add image uploads to on-site theme editor and fix scroll on navigation

Phase 4 of unified editing: image upload handling in hook context.
- Configure uploads in Shop.Page mount for logo, header, icon
- Add upload UI components to theme_editor compact_editor
- Pass uploads through page_renderer to theme editor
- Add cancel_upload handler to PageEditorHook

Also fixes scroll position not resetting on patch navigation:
- Push scroll-top event when path changes in handle_params
- JS listener scrolls window to top instantly

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

PROGRESS.md

@@ -91,7 +91,7 @@ Extend the existing page editor (PageEditorHook + editor_sheet) to include theme
 | 2 | Add 3-tab UI to editor panel (Page/Theme/Settings) | 2h | done |
 | 3 | Extract theme editor into reusable component | 3h | done |
 | 3b | Create settings editor component | 2h | done |
-| 4 | Image upload handling in hook context | 2h | planned |
+| 4 | Image upload handling in hook context | 2h | done |
 | 5 | URL-based mode activation (?edit=theme) | 1h | planned |
 | 6 | Admin routing redirect | 30m | planned |
 | 7 | Polish and testing | 2h | planned |