Add a preference to hide clipboard shortcut hints #1417

Summary

Add a persisted preference that hides the in-window Command/Ctrl shortcut overlays without disabling the shortcuts themselves.

Requirements

• Add shortcuts.showHints with a default of true.
• Keep hints enabled for existing settings files that omit the field.
• Add a Preferences switch with zh-CN and en-US copy.
• Suppress only the KeyHint overlay when disabled; keep the normal icon visible.
• Keep every modifier shortcut operational on macOS and Windows.
• Preserve physical modifier state independently from overlay visibility.

Acceptance criteria

• Holding Command on macOS or Ctrl on Windows shows overlays when enabled.
• No hint overlay appears when the preference is disabled.
• Matching modifier shortcuts continue to trigger their existing actions while hints are disabled.
• Missing-field and explicit-false settings cases are covered by Rust tests.
• Frontend lint/type-check/build and Rust format/Clippy/tests pass on CI.