Add usage statistics tracking with NDJSON storage

[kevintraver]

Summary

Adds a new Stats tab in settings to help users identify their most-used actions and optimize their keyboard shortcut configurations.

Features

• Action tracking: Records every action execution with timestamps and key paths
• Visual analytics: Progress bars show relative usage across all actions
• Hierarchical display: Nested group support with expandable/collapsible UI
• Rich presentation: Shows keyboard shortcuts, app icons, and usage counts inline
• Total count: Displays aggregate execution count across all tracked actions
• Data management: Clear all stats with confirmation dialog

Implementation

• Storage: NDJSON format (newline-delimited JSON) for fast append-only writes and human-readable storage
• Performance: In-memory caching for instant query responses
• UI: SwiftUI hierarchical tree view with proper icon/key badge alignment at all nesting levels
• Integration: Hooks into `Controller.runAction()` to capture full key paths

Stats are stored at `~/Library/Application Support/Leader Key/stats.jsonl`

Screenshot

The Stats pane displays a hierarchical view with:

• Expandable groups showing nested actions
• Visual progress bars indicating relative usage
• Keyboard shortcuts and app icons
• Usage counts for each action and group

Testing

• ✅ Tracks individual action executions
• ✅ Aggregates stats by group (including nested groups)
• ✅ Progress bars scale correctly relative to max count
• ✅ Expand/collapse works for all group levels
• ✅ Icons and shortcuts display correctly for all action types (apps, URLs, commands)
• ✅ Clear stats removes all data and refreshes UI