fix(studio): theme picker confirms applied + surfaces load failures

Two operator-visible fixes prompted by 'I picked a theme but
nothing changed':

  - Theme picker tile taps now show a brief "Theme applied:
    <name>" snackbar — the picker applies themes instantly, so
    the operator needs a confirming cue. Slow `_pluginThemes`
    loads no longer read as "click did nothing".
  - `_pluginThemes` failures (plugin unreachable, manifest
    drift, etc.) write to `FaiLog` so `fai admin doctor` and
    Studio's inline log viewer can surface them. Previously
    swallowed silently, which is what made debugging this so
    miserable.
  - Settings dialog's primary button relabelled to
    "Connect to endpoint" (was "Save & connect") so operators
    don't mistake it for "save my theme choice". The theme
    selection persists at tile-tap time; the Settings dialog
    has nothing left to "save".
  - Theme section header gains an explicit "(applies instantly)"
    cue for the same reason.

Also fixes a pre-existing curly-braces lint in the n8n add-
endpoint dialog. Studio bumped to 0.62.1, editor pinned via
path override at 0.15.1.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/widgets/theme_picker_grid.dart

@@ -109,11 +109,29 @@ class _ThemePickerGridState extends State<ThemePickerGrid> {
       selected: active == null,
       onTap: () {
         StudioApp.of(context)?.setThemePlugin(null);
+        _confirmApplied(l, l.themePluginNone);
         setState(() {});
       },
     );
   }
 
+  /// Brief snackbar acknowledging the theme switch. The picker
+  /// applies themes instantly (no Save), so the operator needs
+  /// a visible cue that the click was registered — without it,
+  /// a slow theme load looks like "the click did nothing".
+  void _confirmApplied(AppLocalizations l, String displayName) {
+    final messenger = ScaffoldMessenger.maybeOf(context);
+    if (messenger == null) return;
+    messenger.removeCurrentSnackBar();
+    messenger.showSnackBar(
+      SnackBar(
+        content: Text(l.themeApplied(displayName)),
+        duration: const Duration(seconds: 2),
+        behavior: SnackBarBehavior.floating,
+      ),
+    );
+  }
+
   Widget _pluginTile(ThemeData theme, String cap, String? active) {
     // Pretty name: studio.theme.sunflower → "Sunflower".
     final display = _displayName(cap);
@@ -138,6 +156,7 @@ class _ThemePickerGridState extends State<ThemePickerGrid> {
       loading: preview == null,
       onTap: () {
         StudioApp.of(context)?.setThemePlugin(cap);
+        _confirmApplied(AppLocalizations.of(context)!, display);
         setState(() {});
       },
     );
@@ -176,6 +195,7 @@ class _ThemePickerGridState extends State<ThemePickerGrid> {
         // Strip alpha component — we only persist the RGB.
         final rgb = hex.substring(2).toUpperCase();
         StudioApp.of(context)?.setThemePlugin('custom:#$rgb');
+        _confirmApplied(l, '#$rgb');
         setState(() {});
       },
     );