feat(#553): move 7-tap Experimental unlock from About row to Build row

Follow-up to #509. Settings is post-connect, so About had no availability edge,
and wrapping the About version subtitle fought the About dialog's onTap. The
7-tap unlock now anchors on the Build (BuildInfo.stamp) row in Device Info,
mirroring Android's build-number gesture; About reverts to a plain row. The
countdown/unlock hint renders inline under the Build row.

Co-located on the #524 test branch so the owner tests one APK; will be split
to its own PR off dev at land time. Agent: QuietSnow (session 31eaba02)

lib/screens/settings_screen.dart

@@ -88,7 +88,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
     });
   }
 
-  void _handleVersionTap() {
+  void _handleUnlockTap() {
     final l10n = context.l10n;
     final settingsService = context.read<AppSettingsService>();
 
@@ -621,10 +621,24 @@ class _SettingsScreenState extends State<SettingsScreen> {
             // #397: identity of the running binary, injected at build time and
             // independent of the marketing version. Its own line so "which build
             // am I on" is answerable at a glance; copyable for bug reports.
+            // Also the hidden 7-tap Experimental unlock anchor (#509/#553),
+            // mirroring Android's Software-info build-number gesture.
             _buildInfoRow(
               l10n.settings_infoBuild,
               BuildInfo.stamp,
               copyValue: BuildInfo.stamp,
+              onTap: _handleUnlockTap,
+            ),
+            if (_versionHint != null)
+              Padding(
+                padding: const EdgeInsets.only(top: 2, bottom: 6),
+                child: Text(
+                  _versionHint!,
+                  style: TextStyle(
+                    fontSize: 12,
+                    color: Theme.of(context).colorScheme.primary,
+                  ),
+                ),
               ),
           ],
         ),
@@ -754,35 +768,11 @@ class _SettingsScreenState extends State<SettingsScreen> {
       child: ListTile(
         leading: const Icon(Icons.info_outline),
         title: Text(l10n.settings_about),
-        // Tapping the version number counts toward the #509 unlock and is
-        // absorbed here; tapping elsewhere on the row still opens the dialog.
-        // Progress is shown inline (below), never as a snackbar.
-        subtitle: GestureDetector(
-          behavior: HitTestBehavior.opaque,
-          onTap: _handleVersionTap,
-          child: Column(
-            crossAxisAlignment: CrossAxisAlignment.start,
-            mainAxisSize: MainAxisSize.min,
-            children: [
-              Text(
+        subtitle: Text(
           l10n.settings_aboutVersion(
             _appVersion.isEmpty ? l10n.common_loading : _appVersion,
           ),
         ),
-              if (_versionHint != null)
-                Padding(
-                  padding: const EdgeInsets.only(top: 2),
-                  child: Text(
-                    _versionHint!,
-                    style: TextStyle(
-                      fontSize: 12,
-                      color: Theme.of(context).colorScheme.primary,
-                    ),
-                  ),
-                ),
-            ],
-          ),
-        ),
         onTap: () => _showAbout(context),
       ),
     );