Allow clearing recording shortcuts

[tkonya]

Summary

Adds the ability to clear any of the three configurable recording shortcuts (Hold to Record, Toggle Recording, and Context Bundler), so users who don't want a given feature bound to a hotkey can remove the binding entirely. Previously these were required to always hold some chord.

Two ways to clear, both standard macOS patterns:

• A small × button rendered inline next to the chord in Settings (visible only when a chord is bound).
• Pressing Delete/Backspace while the recorder is in capture mode (alongside existing Escape to cancel).

Implementation notes

• ChordBindingStore now exposes a tri-state BindingState (unset/cleared/set(KeyChord)) and a dedicated clearBinding(for:). A separate UserDefaults flag marks an action as explicitly cleared, which keeps first-run default fallbacks working — a newly-installed user still gets the default chord, but a user who cleared their binding stays cleared across relaunches.
• AppState's three chord properties become KeyChord?. Cleared actions are simply omitted from the dict handed to HotkeyMonitor, so no hotkey registers for them.
• ShortcutRecorderView accepts KeyChord?, shows "None" when unbound, renders an xmark.circle.fill clear button when bound, and handles Delete during capture to clear (keyCode 51 with no captured keys).

No format or persistence migration needed for existing users — previously-stored chord data continues to decode as .set(chord).

Test plan

• Project builds in Xcode 16+
• ChordBindingStoreTests pass (new coverage for .unset, persisted .cleared, re-set after clear, and "cleared != duplicate")
• Existing GhostPepperTests pass (shortcut-related tests compare KeyChord? against KeyChord — relying on Swift's optional equality)
• Fresh install: defaults still populate for all three shortcuts
• Click × next to a bound shortcut → button shows "None", hotkey stops firing, state persists across relaunch
• Click the chord button, press Delete → same result
• Click the chord button, press Escape → cancels without clearing
• Re-record a chord on a cleared action → works, the previously-bound chord (now cleared) is not treated as a duplicate