[tui] Implement Settings Notification Preferences tab with toggle #200
Description
Implement Settings Notification Preferences tab with toggle
Type: feature
Domain: Codeplane TUI
Implement the Notification Preferences tab (tab 5) within the Settings screen. Features: global notification toggle [ON]/[OFF] with Space/Enter to flip, immediate PUT on toggle (no save button), optimistic toggle with revert on error, [Saving...] state in yellow, [Saved ✓] in green for 2s, read-only notification types list (7 event categories), repository subscriptions count with CLI hint, double-submit prevention via state machine guard, responsive layout. Tests in e2e/tui/settings.test.ts.
Dependencies: tui-settings-screen, tui-settings-data-hooks
Auto-generated by smithers workflow