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/l10n/app_localizations.dart

@@ -641,9 +641,21 @@ abstract class AppLocalizations {
   /// No description provided for @themePluginHeader.
   ///
   /// In en, this message translates to:
-  /// **'Theme plugin'**
+  /// **'Theme plugin (applies instantly)'**
   String get themePluginHeader;
 
+  /// No description provided for @themeApplied.
+  ///
+  /// In en, this message translates to:
+  /// **'Theme applied: {name}'**
+  String themeApplied(String name);
+
+  /// No description provided for @themePluginFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Theme plugin could not load — keeping previous theme.'**
+  String get themePluginFailed;
+
   /// No description provided for @themePluginNone.
   ///
   /// In en, this message translates to:
@@ -725,7 +737,7 @@ abstract class AppLocalizations {
   /// No description provided for @settingsSaveAndConnect.
   ///
   /// In en, this message translates to:
-  /// **'Save & connect'**
+  /// **'Connect to endpoint'**
   String get settingsSaveAndConnect;
 
   /// No description provided for @settingsLanguage.