#1080 feat(desktop): add keyboard shortcut for settings menu

Summary

• Adds ⌘, (Cmd+comma on macOS, Ctrl+comma on Windows/Linux) keyboard shortcut to open the Settings screen
• Adds "Settings..." menu item to the macOS app menu (after About, following Apple HIG)
• Updates the Settings button tooltip to show the correct shortcut

Closes #1069

Test plan

• Press ⌘, on macOS (or Ctrl+, on Windows/Linux) and verify Settings opens
• Click the app menu on macOS and verify "Settings..." menu item appears with the shortcut
• Hover over the Settings gear icon and verify the tooltip shows the correct shortcut
• Verify the shortcut can be customized in Settings > Keyboard Shortcuts

Summary by CodeRabbit

New Features

• Added a keyboard shortcut to quickly access Settings (Cmd+, on macOS; Ctrl+, on Windows and Linux)
• Settings shortcut now displays in the application menu when assigned
• Settings shortcut is categorized under Help in the available hotkeys list

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