feat: Keyboard shortcuts reference dialog

[MaximumTrainer]

Summary

Add an in-app Keyboard Shortcuts dialog listing all keyboard bindings available in the workout player, main window, and workout creator, accessible from the Help menu and via ? key.

Motivation

Keyboard shortcuts for space (pause), arrow keys (interval skip), +/- (difficulty), L (lap), and Esc (quit) exist but are undiscoverable. Users unaware of these shortcuts miss significant productivity gains during workouts.

Acceptance Criteria

• A Keyboard Shortcuts action in the Help menu opens a non-modal dialog
• The ? key opens the dialog from any context (main window, workout player, workout creator)
• The dialog is organised into sections: Workout Player, Workout Creator, Main Window
• Each row shows the key combination and its action description
• All currently-implemented shortcuts are listed (at minimum: Space, →, ←, +, -, L, Esc, F11)
• The dialog is generated from a central ShortcutRegistry data structure (single source of truth) rather than hard-coded UI text, ensuring the dialog and actual bindings stay in sync
• Works on all platforms including WASM (no native menu bar required; accessible via toolbar button as fallback)
• Unit test verifies that every shortcut in ShortcutRegistry maps to a real QShortcut or keyPressEvent handler