v1.5.0

New
- Back / forward between opened documents (Alt+←/→, mouse side buttons, titlebar arrows)
- Settings panel (Ctrl+, / titlebar gear): color scheme, page zoom, TOC default,
  editor options, theme-file restore/disable, version + data directory — all live
- Encoding detection: GBK / Big5 / Shift_JIS / UTF-16 / UTF-8 BOM open correctly
  and save back in their own encoding (UTF-8 fallback, never loses characters)
- Untouched bundled theme in user.css auto-upgrades with new versions
- Read-mode status bar: word count (CJK-aware) + reading time

Fixes
- Unsaved edits were silently lost when opening another file via link / drop / Ctrl+O
- Window could reopen as a tiny "short line" after quitting while minimized
- GitHub alerts, footnote links, Mermaid colors (light app on dark OS),
  dark-mode code-block bands, TOC covering the text
- Visual polish: fitted Mermaid boxes, Chinese UI labels, editor fold markers