…ancellation

- Remove spurious Page404 flash when entering edit mode caused by a
  render tick before useEffect fires (PageEditor)
- Fix data loss when resizing window across mobile/desktop breakpoint:
  MarkdownCodeEditor now uses a resetKey (pageId) to control reinitialization
  instead of initialValue, so typing no longer recreates the CodeMirror instance
- Add isLoading flag and AbortController to pageEditorStore and viewerStore
  so rapid navigation no longer races (stale responses are discarded)
- PageViewer now reads isLoading from viewerStore instead of progressbarStore
- getPageByPath accepts an optional AbortSignal
- Add tests for MarkdownEditor and MarkdownCodeEditor covering the fixed scenarios

…vars)

- Move initialValueRef sync to useLayoutEffect in MarkdownCodeEditor
  (avoids react-hooks/refs error for ref mutation during render)
- Remove markdownRef from MarkdownEditor; pass markdown directly to
  MarkdownCodeEditor (no ref read during render)
- Rename inline mock to MockMarkdownCodeEditor (react-hooks/rules-of-hooks)
- Use useRef snapshot pattern in test to avoid exhaustive-deps warning