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_en.arb

@@ -107,7 +107,14 @@
   "mcpSuggestionMemoryDesc": "Anthropic — persistent knowledge graph for an agent's memory.",
   "mcpSuggestionTimeDesc": "Anthropic — current time + timezone conversion.",
   "mcpServerEnLanguageBadge": "Server text in English — Studio shows it as supplied.",
-  "themePluginHeader": "Theme plugin",
+  "themePluginHeader": "Theme plugin (applies instantly)",
+  "themeApplied": "Theme applied: {name}",
+  "@themeApplied": {
+    "placeholders": {
+      "name": {"type": "String"}
+    }
+  },
+  "themePluginFailed": "Theme plugin could not load — keeping previous theme.",
   "themePluginNone": "Built-in (no plugin)",
   "themePluginHint": "Built-in and plugin tiles ship hand-curated palettes. The Custom tile derives a full Material 3 palette (primary, secondary, tertiary, surface, …) from one seed colour you pick — it's a complete theme, not a single-colour tweak.",
   "themePluginEmpty": "No theme plugins installed. Open the Store and pick one in the studio-plugin category.",
@@ -121,7 +128,7 @@
   "settingsHost": "Host",
   "settingsPort": "Port",
   "settingsTls": "TLS",
-  "settingsSaveAndConnect": "Save & connect",
+  "settingsSaveAndConnect": "Connect to endpoint",
   "settingsLanguage": "Language",
   "settingsLanguageHint": "App language. Affects every page; bilingual store entries also flip via this toggle.",
   "settingsHubEndpointHint": "Where should Studio connect? Default is the local hub at 127.0.0.1:50051.",