feat: Allow multiple keyboard shortcuts per action

[nino-chavez]

Summary

• Adds ability to assign multiple keyboard shortcuts to any action
• Hover over a shortcut row to reveal:
  • + button: Add an additional shortcut for that action
  • x button: Remove individual shortcuts (only when more than one exists)
• Click any existing shortcut key to edit/replace just that specific shortcut (preserves others)
• Updated dialog description to explain new functionality

Screenshots

When hovering over a shortcut row:

• The + button appears on the right to add new shortcuts
• The x button appears on each key (if >1 exists) to remove individual shortcuts

Test plan

• Open Keyboard Shortcuts dialog
• Hover over "Play/Pause" (has Space and K) → verify + and x buttons appear
• Click + to add a new shortcut (e.g., press "P") → verify it adds alongside existing ones
• Click x on one shortcut → verify only that one is removed
• Click an existing shortcut key to edit it → verify only that key changes, others remain
• Verify cannot remove the last shortcut (shows error toast)
• Click Reset to Default → verify all shortcuts reset

Closes #627

🤖 Generated with Claude Code

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[nino-chavez]

Closing: upstream codebase overhaul (#697) deleted/restructured the files this PR modified. Needs reimplementation against the new codebase.