Add mouse gesture practice mode and cheatsheet hints

[multiplex55]

Motivation

• Provide a practice/training mode for mouse gestures so users can try gestures without triggering actions.
• Surface quick in-context guidance when RMB is held briefly (cheat sheet) and give a closest-match suggestion on misses to help learn gestures.

Description

• Added a practice_mode boolean to MouseGestureSettings and MouseGestureConfig with default false, and wired it through runtime application logic in src/plugins/mouse_gestures.rs.
• Exposed a launcher command mg practice that toggles practice mode and displays a toast; the toggle is implemented in toggle_practice_mode() and handled in src/gui/mod.rs.
• Updated the worker loop in src/mouse_gestures/service.rs to suppress emitting WatchEvent::ExecuteAction when practice_mode is active while still logging matches/misses with tracing::info!.
• Implemented a hold-to-cheatsheet hint: when RMB is held but still below deadzone for ~250ms the HintOverlay shows a multi-line cheat sheet (top gestures by iteration) and hides when the deadzone is exceeded; reused HintOverlay for multi-line display.
• Added a simple "closest match" suggestion for misses by exposing the first entry from candidate_matches with a fuzzy match-type label in the hint text.
• Minor supporting helpers: format_cheatsheet_text, match_type_label, constants for cheat-sheet delay/limit, and small hint formatting adjustments.

Testing

• Added unit tests in tests/mouse_gestures_service.rs: practice_mode_suppresses_execute_action and cheat_sheet_overlay_shows_after_delay_without_tokens, and updated hint_text_includes_best_guess_and_match_type expectation to include the closest-match line.
• Attempted to run the test suite with cargo test --test mouse_gestures_service, but the run failed during build due to a missing system dependency: the alsa pkg-config entry required by alsa-sys was not available in the environment (pkg-config / alsa.pc not found), so tests did not complete.

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Codex Task