Bug: Quiet hours settings crash on save and aren't restored on startup

[Orinks]

Bug Description

The settings dialog references two missing members on ClockService:

• set_quiet_hours(start_time, end_time) method (called in settings_dialog.py line 427)
• quiet_hours_enabled attribute (referenced in settings_dialog.py line 432)

Neither exists in services/clock_service.py, which means:

1. Saving quiet hours settings crashes with AttributeError
2. Quiet hours aren't restored on startup — app.py._load_config() and save_config() don't handle quiet hours config keys

Steps to Reproduce

1. Open Settings → Quiet Hours tab
2. Enable quiet hours
3. Click OK → crash

Fix

• Add set_quiet_hours() method and quiet_hours_enabled property to ClockService
• Add quiet hours persistence to app.py._load_config() and save_config()
• Add tests for the new method

[Orinks]

Fixed in PR #20 (commit 03347c7)