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>

docs/plans/unified-editing-mode.md

@@ -1,6 +1,6 @@
 # Unified On-Site Editing Mode
 
-Status: Planned
+Status: In Progress (Phase 1-2 complete)
 
 Replace the separate admin theme editor (`/admin/theme`) with an on-site editing experience. When the admin clicks "Theme" or "Edit page", they're taken to the actual shop where a sliding panel allows them to edit either page content or theme settings — seeing changes live on the real site.