Add shortcut customization for accepting suggestions

[FuJacob]

Problem

Tabby currently uses Tab as the primary accept action. That is simple, but some apps already rely heavily on Tab, and some users may prefer a different key or key chord.

Goal

Allow users to customize the accept shortcut while preserving safe defaults and avoiding common host-app conflicts.

Proposed Scope

• Add a Shortcuts section in Settings.
• Show the current accept shortcut and explain what it does.
• Allow recording a new shortcut or restoring the default Tab behavior.
• Validate shortcuts and warn about risky choices.
• Update InputMonitor and acceptance logic to use the configured shortcut.
• Ensure pass-through behavior remains correct when the shortcut should not be consumed.

Acceptance Criteria

• Users can view and change the accept shortcut in Settings.
• The configured shortcut persists across app restarts.
• The input monitor recognizes the configured shortcut instead of hard-coding only Tab.
• Tabby still passes input through when no valid suggestion is ready.
• Users can reset back to the default shortcut.

Open Questions

• Should single-key shortcuts other than Tab be allowed?
• Should modifier chords such as Option+Tab or Control+Space be preferred?
• Should shortcuts be global or allow per-app overrides?
• How should conflicts with system shortcuts or host-app shortcuts be surfaced?

[FuJacob]

Done