fix(hotkeys): support F13–F20 and the native Mac Dictation key

[Eclock2000]

Summary

Support F13–F20 and add the native Mac microphone/Dictation key to the existing dictation shortcut menu. Selecting it replaces the current binding; there is still one dictation trigger.

Changes

• Accept F13–F20 in shared validation, platform key mappings, and frontend capture, including WebKit private-use function-key characters.
• On macOS, route native Dictation-key press/release events into the existing recording modes. Consume only an unmodified native press and its paired repeats/release. Ordinary F5, F20, and modified native presses pass through; capture does not begin during Secure Event Input.
• Register the selected key through the existing settings transaction. Failed registration retains the previous binding and displays an error. Switching away or quitting releases native interception.
• Preserve the existing side-aware event channel during failed-switch rollback.

The native option requires a compatible Mac keyboard and Accessibility permission. There is no setup wizard, extra trigger, saved fallback binding, status polling, or dedicated retry UI. The shared preference schema and hotkey event loops are unchanged.

Validation

• TypeScript/Vite production build and frontend recorder tests passed.
• 36 exact-source Rust harness tests passed for key parsing, native-key state transitions, and side-aware routing, including failed-switch restoration.
• 11 Core settings transaction tests passed, covering runtime and persistence failures.
• Final macOS arm64 application build and ad hoc signature verification passed (npm run tauri -- build --bundles app; codesign --verify --deep --strict).
• Earlier stable-backport hardware checks confirmed F20 through a remap and native keycode 176 interception without opening macOS Dictation; Fn+F5 remained ordinary F5.

Integrated beta recording/text insertion, restart behavior, and Windows hardware behavior have not been manually verified. These limitations are disclosed for review; the earlier hardware checks are not full beta application acceptance.