Add configurable input status hints

[hitsmaxft]

Extend the existing input status panel with opt-in modifier and focus hints. In non-ASCII mode, focusing a text field shows the active schema name until the first keydown or a configurable timeout (2 seconds by default). Held Control/Option/Shift/Command keys remain visible while pressed. When name hints are enabled, switching back to non-ASCII mode also shows the active schema name instead of the usual Chinese-mode label; the English-mode label is preserved.

Both features default off. Enable them in squirrel.custom.yaml and redeploy:

patch:
  "modifier_hint/show": true
  "focus_hint/show": true
  "focus_hint/duration": 2

All hints share the existing Chinese/English status component and use the candidate font and theme. No extra window or font option is introduced. With candidates visible, modifier hints appear in a status row below the candidates, outside candidate hit testing. Status-message expiry clears only the notification, preserving held-key indicators.

The implementation separates transient state from rendering, polls only while input hints are visible, and clears them on deactivation or input-source changes. README documents configuration and client requirements. CI runs the standalone state tests on feature branches containing slashes.

Validation:

• Local state tests and Swift type checking passed. Tests cover all modifier combinations, focus timeout/dismissal, ASCII-mode suppression, notification expiry, and message deduplication.
• Fork CI passed: state tests, SwiftLint, universal Xcode build, installer packaging, and unused-code scan.
• The merged-panel version was installed on macOS. The updated mode-switch name label awaits another UI check.

Hints require valid client-provided caret geometry and input activation callbacks. Modifier indicators exclude Caps Lock and Fn.

[hitsmaxft]