fix(#684): apply pre-PR review, never show the notice while disconnected

Gemini gate (2.5-pro) findings, verified against source before acting.

ACCEPTED, real defect: the notice would have appeared on every disconnect.
`_activeTransport` defaults to bluetooth and is reset to it on teardown, the
device is null while disconnected so `mtuNow` is unavailable, and
`effectiveMaxFrameSize` therefore returns the same 20-byte floor an unreported
MTU produces. Neither chat screen pops on disconnect, so the composer stays
built. The banner would have blamed the link for a small packet size when there
was no link. Now gated on `isConnected`.

ACCEPTED in substance: a permanent error-red banner over a link that
legitimately fits only a few characters is noise, and noise is what teaches
people to ignore the banner that matters. Kept the 16-byte threshold, since
lowering it would let the misleading 4-byte case through, and split the
severity instead: 0 bytes is an error, 1-15 is informational.

REJECTED, unfounded: a semanticLabel on the icon. Flutter's Icon wraps its
glyph in ExcludeSemantics and only announces a label when one is given
(icon.dart:348-356), so an unlabelled icon is already decorative, and the two
Text children carry the whole message. Comment added so it is not "fixed"
later.

analyze clean, format clean, 962 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/screens/channel_chat_screen.dart

@@ -1435,6 +1435,7 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
         ComposerBudgetNotice(
           maxBytes: maxBytes,
           transport: connector.activeTransport,
+          isConnected: connector.isConnected,
         ),
         if (_replyingToMessage != null)
           Builder(

lib/screens/chat_screen.dart

@@ -609,6 +609,7 @@ class _ChatScreenState extends State<ChatScreen> {
         ComposerBudgetNotice(
           maxBytes: maxBytes,
           transport: connector.activeTransport,
+          isConnected: connector.isConnected,
         ),
         _buildComposerBar(context, colorScheme, settings, maxBytes, connector),
       ],

lib/widgets/composer_budget_notice.dart

@@ -28,23 +28,39 @@ class ComposerBudgetNotice extends StatelessWidget {
   /// as generic failure.
   final MeshCoreTransportType transport;
 
+  /// Whether a radio is actually connected.
+  ///
+  /// Required, and not cosmetic. While disconnected, `_activeTransport` still
+  /// reads `bluetooth` (its default) and there is no device, so the frame
+  /// budget falls to the same 20-byte floor an unreported MTU produces. Without
+  /// this gate the notice would appear on every disconnect and blame the link
+  /// for a small packet size when there is no link at all.
+  final bool isConnected;
+
   const ComposerBudgetNotice({
     super.key,
     required this.maxBytes,
     required this.transport,
+    required this.isConnected,
   });
 
   @override
   Widget build(BuildContext context) {
+    if (!isConnected) return const SizedBox.shrink();
     if (isComposerBudgetUsable(maxBytes)) return const SizedBox.shrink();
 
     final l10n = context.l10n;
     final scheme = Theme.of(context).colorScheme;
 
     // A budget of 0 blocks every keystroke; 1-15 accepts input but cannot carry
-    // a sentence. The user needs those described differently: the first is
-    // "you cannot send", the second is "you have almost no room".
-    final title = maxBytes <= 0
+    // a sentence. Those are different severities and must not look the same.
+    // Zero is a failure: nothing can be sent. Non-zero is a fact about a
+    // working link, so it is stated rather than alarmed about - a permanent red
+    // banner over a link that genuinely only fits a few characters is noise,
+    // and noise is what teaches people to ignore the banner that matters.
+    final blocked = maxBytes <= 0;
+
+    final title = blocked
         ? l10n.composerBudgetBlockedTitle
         : l10n.composerBudgetLimitedTitle(maxBytes);
 
@@ -52,14 +68,30 @@ class ComposerBudgetNotice extends StatelessWidget {
         ? l10n.composerBudgetBluetoothCause
         : l10n.composerBudgetGenericCause;
 
+    final background = blocked
+        ? scheme.errorContainer
+        : scheme.secondaryContainer;
+    final foreground = blocked
+        ? scheme.onErrorContainer
+        : scheme.onSecondaryContainer;
+
     return Container(
       width: double.infinity,
       padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 10),
-      color: scheme.errorContainer,
+      color: background,
       child: Row(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
-          Icon(Icons.error_outline, size: 18, color: scheme.onErrorContainer),
+          // No semanticLabel on purpose: Icon wraps its glyph in
+          // ExcludeSemantics and only announces a label when given one, so an
+          // unlabelled icon is correctly decorative here. The two Text children
+          // below already carry the whole message to a screen reader, and
+          // labelling the icon too would just repeat it.
+          Icon(
+            blocked ? Icons.error_outline : Icons.info_outline,
+            size: 18,
+            color: foreground,
+          ),
           const SizedBox(width: 10),
           Expanded(
             child: Column(
@@ -69,17 +101,11 @@ class ComposerBudgetNotice extends StatelessWidget {
                   title,
                   style: TextStyle(
                     fontWeight: FontWeight.w600,
-                    color: scheme.onErrorContainer,
+                    color: foreground,
                   ),
                 ),
                 const SizedBox(height: 2),
-                Text(
-                  cause,
-                  style: TextStyle(
-                    fontSize: 12,
-                    color: scheme.onErrorContainer,
-                  ),
-                ),
+                Text(cause, style: TextStyle(fontSize: 12, color: foreground)),
               ],
             ),
           ),