feat: add terminal bell setting to enable/disable audio notifications

[afarber]

TLDR

Adds a user-facing general.terminalBell setting that lets users toggle the terminal bell notification on/off via the settings dialog. The terminal bell feature already existed but wasn't configurable - this PR wires up the setting so users can disable it if they find it annoying.

Dive Deeper

The terminal bell (\u0007) was already being emitted by notifyTerminalAttention() when:

• Waiting for tool approval (and terminal is unfocused)
• A long task (>20 seconds) completes (and terminal is unfocused)

The changes are minimal:

1. Added terminalBell boolean to settings schema with showInDialog: true
2. Added enabled option to TerminalNotificationOptions interface
3. Hook now reads the setting and passes it through to the notification function
4. Added test coverage for the disabled case

Default is true to preserve existing behavior.

Reviewer Test Plan

1. Run npm run build && npm run start
2. Type /settings and verify "Terminal Bell" toggle appears under General category
3. With setting enabled (default):

• Ask qwen to edit a file that requires approval
• Switch to another terminal tab
• Verify you hear/see the bell when approval is requested

4. Disable the setting in /settings
5. Repeat step 3 - verify no bell sounds
6. Run npx vitest run src/ui/hooks/useAttentionNotifications.test.ts src/utils/attentionNotification.test.ts - all 10 tests should pass

Testing Matrix

	🍏	🪟	🐧
npm run	yes	❓	yes
npx	❓	❓	❓
Docker	❓	❓	❓
Podman	❓	-	-
Seatbelt	❓	-	-

Linked issues / bugs

Closes #1193

[afarber]

Smoke tested on macOS 26.1:

[BlockHand]

@afarber Thank you for your contribution! I've already tested it and everything is fine. We will merge this PR.