feat: add configurable macOS selection shortcut

[fly1d]

Summary

• default selection translation to Option + K
• let desktop users disable or record a custom global shortcut, including while disabled
• persist preferences locally and surface registration, rollback, capture, focus, and permission failures
• serialize shortcut replacement and selected-text processing to protect clipboard fallback
• update product and build documentation for the new shortcut and privacy boundary

Risk

• Low: documentation, copy, styles, or isolated dictionary data
• Medium: UI behavior, translation provider, document parsing, or shared API
• High: selection capture, OS permissions, credentials, packaging, or releases

Verification

• npm run check (34 tests, typecheck, production build)
• npm run test:smoke (5 passed, 1 desktop-only viewport test skipped by design)
• npm run test:site (12 passed)
• npm run desktop:check (Rust fmt, Clippy, 8 tests, cargo check)
• release .app build
• real app UI smoke: default, disable, edit while disabled, re-enable, persistence across restart, reset to default
• No API keys, user content, or sensitive logs were added
• Error and fallback states were exercised in unit tests

Review Notes

Bare K is intentionally rejected because a Carbon global shortcut would consume normal typing system-wide. The default is Option + K; printable keys and F1-F12 require Command, Control, or Option.

The real .app UI flow was exercised on macOS. A human still needs to press the global shortcut from another application with Accessibility allowed and denied; Computer Use cannot synthesize system-wide shortcuts or approve privacy permissions. Known clipboard fallback limits remain documented.