feat(whisper): add dropdown preset keybinding selector for speech-to-text

[monotykamary]

Summary

There was no easy way to change the keybinding for speech-to-text beyond the default Fn key. The HotkeyRecorder component cannot capture standalone modifier keys (Option, Command, etc.) because browser keydown events treat them as modifiers rather than keys, making them impossible to record.

This PR adds a dropdown with preset keybinding options in the Whisper hotkey settings, with full backend support for standalone modifier keys as hold-to-talk triggers.

Changes

UI — src/renderer/src/settings/AITab.tsx

• Added a <select> dropdown with 5 hold-to-talk preset options:
  • fn (hold-to-talk, default)
  • ⌥ Left Option (hold-to-talk)
  • ⌥ Right Option (hold-to-talk)
  • ⌘ Left Command (hold-to-talk)
  • ⌘ Right Command (hold-to-talk)
• A "Custom…" option reveals the HotkeyRecorder for recording any key combo
• Replaced the old HotkeyRecorder + "Set to Fn" button UI
• Added always-visible hint text explaining the hold-to-talk behavior

UI — src/renderer/src/settings/HotkeyRecorder.tsx

• Added autoRecord prop: when set, the recorder auto-starts in recording mode and grabs focus on mount
• Fixes the "Custom…" flow — previously the <select> retained focus and intercepted keyboard events, preventing the recorder from receiving input

Display — src/renderer/src/utils/hyper-key.ts

• Added directional display for left/right modifier variants: ⌥←, ⌥→, ⌘←, ⌘→
• Added these to the modifier symbols set so they render as modifiers (not keys)

Backend — src/main/main.ts

• Added STANDALONE_MODIFIER_KEYCODES map with both left and right modifier key codes (alt→58, leftoption→58, rightoption→61, command→55, leftcommand→55, rightcommand→54, control→59, shift→56 — macOS virtual key codes)
• Added isStandaloneModifierShortcut() and needsNativeHoldWatcher() helper functions
• Extended parseHoldShortcutConfig() key code map with left/right modifier keys, setting the corresponding modifier flag when the key token is a standalone modifier (matching the existing Fn pattern)
• Added fnSpeakToggleCurrentShortcut variable (was previously hardcoded to 'Fn'), so the watcher adapts to whichever key the user selects
• Generalized syncFnSpeakToggleWatcher() to activate for any shortcut needing a native watcher (Fn, Alt/Option, left or right) and properly restart when the shortcut changes
• Updated registerCommandHotkeys() and updateCommandHotkey IPC handler to skip Electron globalShortcut for standalone modifier shortcuts on the speak-toggle command (handled by native CGEventTap instead)
• Updated normalizeAccelerator() to recognize left/right modifier tokens as modifiers when used in combos (e.g. LeftOption+Space → Alt+Space)
• Standalone modifier shortcuts are rejected for non-speak-toggle commands with "unavailable" error

No native code changes needed

The hotkey-hold-monitor.swift binary already handles flagsChanged events for modifier keys and accepts any key code + modifier combination via CLI args.

Locales — all 9 locale files

• Removed: setToFn, fnHint, fnHintBefore, fnHintAfter
• Added: custom ("Custom…"), holdToTalkHint (explanatory help text)
• All locales verified in structural parity with en.json

How to Test

1. Open Settings → AI → Whisper
2. Under Whisper Hotkeys, verify the dropdown appears with 5 preset options + Custom
3. Select ⌥ Left Option — the watcher should start monitoring the left Option key
4. Hold left Option to start dictation, release to stop — verify it works like Fn
5. Select ⌘ Right Command — the watcher should start monitoring the right Command key
6. Hold right Command to start dictation, release to stop
7. Select Custom… — the HotkeyRecorder should auto-start recording; press a key combo
8. Verify the whisper overlay shortcut hint label updates correctly (shows ⌥←, ⌥→, ⌘←, ⌘→ for left/right variants)
9. Verify selecting a standalone modifier for a non-whisper command shows "hotkey unavailable"