Move plugin settings access from sidebar to app card

Remove the "Extensions" section from the sidebar nav. Instead, each app
card on the Apps page shows an "Extension" button when the current user
has access to that app's plugin (matched by service ID). The button links
to /settings/plugins/:id alongside the existing admin Settings button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -169,7 +169,7 @@ docker compose up --build -d
     │   │   └── useTheme.ts                ← Theme toggle
     │   ├── components/
     │   │   ├── AppShell.tsx               ← Layout: Sidebar + scrollable main
-    │   │   ├── Sidebar.tsx                ← Collapsible nav; "Extensions" section auto-populated from /api/plugins
+    │   │   ├── Sidebar.tsx                ← Collapsible nav (icons ↔ icons+labels)
     │   │   ├── ThemeToggle.tsx            ← Light/dark mode toggle
     │   │   ├── PluginSchemaForm.tsx       ← JSON Schema → React form (boolean/string/number/readOnly)
     │   │   └── ui/                        ← shadcn/ui components (Button, Input, …)