implement unified on-site editor phases 1-2

Add theme editing to the existing PageEditorHook, enabling on-site
theme customisation alongside page editing. The editor panel now has
three tabs (Page, Theme, Settings) and can be collapsed while
keeping editing state intact.

- Add theme editing state and event handlers to PageEditorHook
- Add 3-tab UI with tab switching logic
- Add transparent overlay for click-outside dismiss
- Add mobile drag-to-resize with height persistence
- Fix animation replay on drag release (has-dragged class)
- Preserve panel height across LiveView re-renders
- Default to Page tab on editable pages, Theme otherwise
- Show unsaved changes indicator on FAB when panel collapsed
- Fix handle_event grouping warning in admin theme

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

PROGRESS.md

@@ -87,8 +87,8 @@ Extend the existing page editor (PageEditorHook + editor_sheet) to include theme
 
 | Phase | Description | Est | Status |
 |---|---|---|---|
-| 1 | Add theme editing state to PageEditorHook | 2h | planned |
-| 2 | Add 3-tab UI to editor panel (Page/Theme/Settings) | 2h | planned |
+| 1 | Add theme editing state to PageEditorHook | 2h | done |
+| 2 | Add 3-tab UI to editor panel (Page/Theme/Settings) | 2h | done |
 | 3 | Extract theme editor into reusable component | 3h | planned |
 | 3b | Create settings editor component | 2h | planned |
 | 4 | Image upload handling in hook context | 2h | planned |