fix(#80): broker enable/disable feedback + don't validate a disable

On-hardware test feedback:
- Long-press Enable/Disable was silent on success AND failure. It now confirms
  on success and, on failure, surfaces the device's reason (lastError) instead
  of nothing.
- The client now refuses to quick-Enable a broker that can't work (incomplete
  required fields, e.g. JWT without audience/owner) and says why -- so it never
  reports "enabled" on a broker that won't run. (Firmware should reject too.)
- The editor blocked a DISABLE on blank/invalid fields. Validation now gates
  ENABLING only -- disabling a slot with partial values is fine; it won't be
  active.

Validation rules shared via BrokerConfig.enableError (editor pre-save + list
quick-Enable). Full suite green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/models/observer_config.dart

@@ -144,6 +144,19 @@ class BrokerConfig {
   /// A slot is occupied iff it has a URL (firmware `cfg.url[0] != '\0'`).
   bool get isPopulated => url.isNotEmpty;
 
+  /// Why this broker can't be safely enabled, or null if complete. Shared by
+  /// the editor's pre-save check and the list's quick Enable so the client
+  /// never activates a broker that can't work (#80).
+  String? get enableError {
+    if (url.isEmpty) return 'URL is required';
+    if (port < 1 || port > 65535) return 'Port must be between 1 and 65535';
+    if (authType == BrokerAuthType.jwt &&
+        (jwtAudience.isEmpty || jwtOwner.isEmpty)) {
+      return 'JWT auth needs an audience and an owner';
+    }
+    return null;
+  }
+
   /// Build from one slot's decoded `key=value` lines (the OCFG_BROKER_KV bodies).
   /// Unknown keys are ignored; missing keys keep the defaults.
   factory BrokerConfig.fromWireFields(int slot, Map<String, String> kv) {

lib/screens/settings/broker_editor_screen.dart

@@ -112,20 +112,16 @@ class _BrokerEditorScreenState extends State<BrokerEditorScreen> {
   bool get _dirty =>
       _changedFields().isNotEmpty || _enabled != _baseline.enabled;
 
-  /// Reject predictably-bad values before they reach the wire (#80). Returns an
-  /// error message, or null when the form is valid.
-  String? _validate() {
-    if (_url.text.trim().isEmpty) return 'URL is required';
-    final port = int.tryParse(_port.text.trim());
-    if (port == null || port < 1 || port > 65535) {
-      return 'Port must be between 1 and 65535';
-    }
-    if (_authType == BrokerAuthType.jwt &&
-        (_jwtAudience.text.trim().isEmpty || _jwtOwner.text.trim().isEmpty)) {
-      return 'JWT auth requires an audience and an owner';
-    }
-    return null;
-  }
+  /// Pre-enable validation, shared with the list's quick Enable via
+  /// [BrokerConfig.enableError]. Builds a tentative config from the form.
+  String? _validate() => BrokerConfig(
+    slot: _baseline.slot,
+    url: _url.text.trim(),
+    port: int.tryParse(_port.text.trim()) ?? -1,
+    authType: _authType,
+    jwtAudience: _jwtAudience.text.trim(),
+    jwtOwner: _jwtOwner.text.trim(),
+  ).enableError;
 
   void _snack(String msg, {bool isError = false}) {
     ScaffoldMessenger.of(context).showSnackBar(
@@ -137,10 +133,14 @@ class _BrokerEditorScreenState extends State<BrokerEditorScreen> {
   }
 
   Future<void> _save() async {
-    final err = _validate();
-    if (err != null) {
-      _snack(err, isError: true);
-      return;
+    // Validation only gates ENABLING — disabling a slot with blank/partial
+    // fields is fine; it won't be active.
+    if (_enabled) {
+      final err = _validate();
+      if (err != null) {
+        _snack(err, isError: true);
+        return;
+      }
     }
     final svc = context.read<ObserverConfigService>();
     final messenger = ScaffoldMessenger.of(context);

lib/screens/settings/mqtt_brokers_screen.dart

@@ -57,9 +57,43 @@ class _MqttBrokersScreenState extends State<MqttBrokersScreen> {
   }
 
   Future<void> _quickToggle(BrokerConfig b, bool enable) async {
+    final messenger = ScaffoldMessenger.of(context);
+    final errorColor = Theme.of(context).colorScheme.error;
+    // The client refuses to enable a broker that can't work, and says why
+    // (firmware should reject it too, but don't claim success on the wire).
+    if (enable) {
+      final err = b.enableError;
+      if (err != null) {
+        messenger.showSnackBar(
+          SnackBar(
+            content: Text("Can't enable broker ${b.slot}: $err"),
+            backgroundColor: errorColor,
+          ),
+        );
+        return;
+      }
+    }
     final svc = context.read<ObserverConfigService>();
-    await svc.setBrokerField(b.slot, 'enabled', enable ? '1' : '0');
-    if (mounted) await _reload();
+    final ok = await svc.setBrokerField(b.slot, 'enabled', enable ? '1' : '0');
+    if (!mounted) return;
+    if (ok) {
+      messenger.showSnackBar(
+        SnackBar(
+          content: Text('Broker ${b.slot} ${enable ? 'enabled' : 'disabled'}'),
+        ),
+      );
+      await _reload();
+    } else {
+      messenger.showSnackBar(
+        SnackBar(
+          content: Text(
+            svc.lastError ??
+                'Failed to ${enable ? 'enable' : 'disable'} broker ${b.slot}',
+          ),
+          backgroundColor: errorColor,
+        ),
+      );
+    }
   }
 
   Future<void> _clear(BrokerConfig b) async {

test/screens/broker_editor_screen_test.dart

@@ -115,7 +115,7 @@ void main() {
     await _open(
       tester,
       fake,
-      const BrokerConfig(slot: 1, url: 'h', port: 1883),
+      const BrokerConfig(slot: 1, url: 'h', port: 1883, enabled: true),
     );
 
     await tester.enterText(find.byKey(const Key('broker_port')), '99999');
@@ -172,4 +172,57 @@ void main() {
     expect(find.text('fromdevice'), findsOneWidget);
     expect(find.text('8883'), findsOneWidget);
   });
+
+  testWidgets('disabling a slot saves even with incomplete fields', (
+    tester,
+  ) async {
+    final fake = _FakeSvc();
+    await _open(
+      tester,
+      fake,
+      const BrokerConfig(
+        slot: 2,
+        url: 'h',
+        port: 1883,
+        enabled: true,
+        authType: BrokerAuthType.jwt, // blank audience/owner
+      ),
+    );
+
+    await tester.tap(find.byType(SwitchListTile)); // toggle enabled OFF
+    await tester.pumpAndSettle();
+    await tester.tap(find.byTooltip('Save'));
+    await tester.pumpAndSettle();
+
+    expect(
+      fake.saveCalls,
+      hasLength(1),
+      reason: 'disabling must not be blocked by field validation',
+    );
+    expect(fake.saveCalls.single.enable, isFalse);
+    await _drainSnack(tester);
+  });
+
+  testWidgets('enabling with incomplete JWT is blocked', (tester) async {
+    final fake = _FakeSvc();
+    await _open(
+      tester,
+      fake,
+      const BrokerConfig(
+        slot: 2,
+        url: 'h',
+        port: 1883,
+        authType: BrokerAuthType.jwt, // disabled, blank audience/owner
+      ),
+    );
+
+    await tester.tap(find.byType(SwitchListTile)); // toggle enabled ON
+    await tester.pumpAndSettle();
+    await tester.tap(find.byTooltip('Save'));
+    await tester.pumpAndSettle();
+
+    expect(fake.saveCalls, isEmpty);
+    expect(find.textContaining('needs an audience'), findsOneWidget);
+    await _drainSnack(tester);
+  });
 }

test/screens/mqtt_brokers_screen_test.dart

@@ -18,7 +18,11 @@ class _FakeSvc extends ObserverConfigService {
   final List<BrokerConfig> _brokers;
   final List<String> setCalls = [];
   final List<int> clearCalls = [];
+  bool toggleOk = true;
+  String? errorText;
 
+  @override
+  String? get lastError => errorText;
   @override
   ObserverConfig? get config => ObserverConfig(brokers: _brokers);
   @override
@@ -26,7 +30,7 @@ class _FakeSvc extends ObserverConfigService {
   @override
   Future<bool> setBrokerField(int slot, String field, String value) async {
     setCalls.add('$slot.$field=$value');
-    return true;
+    return toggleOk;
   }
 
   @override
@@ -97,4 +101,46 @@ void main() {
     await tester.pumpAndSettle();
     expect(fake.clearCalls, contains(2));
   });
+
+  testWidgets('quick Enable refuses an incomplete broker with a reason', (
+    tester,
+  ) async {
+    final fake = _FakeSvc(const [
+      BrokerConfig(slot: 2, url: 'a', port: 1883, authType: BrokerAuthType.jwt),
+    ]);
+    await _pump(tester, fake);
+
+    await tester.longPress(find.text('[2] a'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Enable'));
+    await tester.pumpAndSettle();
+
+    expect(
+      fake.setCalls,
+      isEmpty,
+      reason: 'an incomplete broker must not be enabled',
+    );
+    expect(find.textContaining("Can't enable"), findsOneWidget);
+    await tester.pump(const Duration(seconds: 5));
+  });
+
+  testWidgets('a failed quick toggle surfaces the device reason', (
+    tester,
+  ) async {
+    final fake =
+        _FakeSvc(const [
+            BrokerConfig(slot: 2, url: 'a', port: 1883, enabled: true),
+          ])
+          ..toggleOk = false
+          ..errorText = 'ERROR broker busy';
+    await _pump(tester, fake);
+
+    await tester.longPress(find.text('[2] a'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Disable'));
+    await tester.pumpAndSettle();
+
+    expect(find.textContaining('ERROR broker busy'), findsOneWidget);
+    await tester.pump(const Duration(seconds: 5));
+  });
 }