feat(ui): add app-level zoom

[yicheng47]

Closes #295.

Summary

• add global 80–150% app zoom with Cmd/Ctrl shortcuts and Appearance settings controls
• restore and persist zoom across library and reader windows without an unzoomed launch flash
• keep PDF content zoom on Cmd/Ctrl+Shift +/- and preserve its per-book controls
• reposition macOS traffic lights as the overlay titlebar scales

Verification

• npm test
• npm exec tsc -- --noEmit
• npm run build
• npm run lint (0 errors; 8 pre-existing warnings)
• cargo check
• cargo test
• cargo clippy -- -D warnings
• manual smoke test across library, EPUB, PDF, persistence, and fullscreen