redirect /admin/theme to on-site editor at /?edit=theme

- Phase 5 was already implemented (URL mode activation via ?edit param)
- Phase 6: Add RedirectController to redirect /admin/theme → /?edit=theme
- Update admin sidebar and dashboard links to point directly to /?edit=theme
- Delete old Admin.Theme.Index LiveView and template (no longer needed)
- Update tests for new redirect behavior

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PROGRESS.md

@@ -92,8 +92,8 @@ Extend the existing page editor (PageEditorHook + editor_sheet) to include theme
 | 3 | Extract theme editor into reusable component | 3h | done |
 | 3b | Create settings editor component | 2h | done |
 | 4 | Image upload handling in hook context | 2h | done |
-| 5 | URL-based mode activation (?edit=theme) | 1h | planned |
-| 6 | Admin routing redirect | 30m | planned |
+| 5 | URL-based mode activation (?edit=theme) | 1h | done |
+| 6 | Admin routing redirect | 30m | done |
 | 7 | Polish and testing | 2h | planned |
 
 ### Quick fixes (from usability testing)