Phase 4.6: Settings modal and Appearance UI (+ Phase 4.7 gate)

[danielmrdev]

Summary

Closes #32 and #33.

• Phase 4.6 (Phase 4.6: Settings modal and Appearance UI #32): Settings modal opened with Ctrl+Comma (Cmd+Comma on macOS) over the inert Writing surface. Visible sidebar with the single labelled, Phosphor Duotone-iconed Appearance category. Appearance controls — Theme mode (System/Paper/Ink), Writing width, Letter spacing, Font size (12–24 px), Typeface, Body weight, Document-title weight — apply live and persist immediately; System light/dark mapping controls stay hidden while each appearance has one valid Theme. Typeface selection shows Downloading, applies only after verified bytes register, and offers Retry on failure; a failed save keeps the visible change active with retryable feedback in Settings. Selecting the already-active Typeface or applying no change is a no-op. Escape or overlay click closes Settings restoring editor focus and caret.
• Phase 4.7 (Phase 4.7: Cross-platform settings gate #33): Cross-platform settings gate record in docs/phase-4-cross-platform-verification.md — automated gate, native platform coverage, launch smoke coverage, manual verification record (all cells unverified until executed directly per platform), scope guard, and explicit limits.

Verification

• Local Linux gate: cargo fmt --check, cargo build --locked --all-targets, cargo clippy --locked --all-targets --all-features -- -D warnings (clean), full test suite (197 + 36 unit + 2 smoke pass; reference workload and manifest verification pass explicitly).
• macOS/Windows automated gate runs in this PR's CI.