feat: add alarm with shake animation and tabbed settings

[Svtter]

Summary

• Add alarm support: the clock window shakes for 15 seconds when an alarm time is reached; clicking the clock stops the shake early
• Reorganize the settings dialog into a TabControl with two tabs: General (auto-start, second hand, log folder) and Alarms
• Support multiple alarms, each with enable toggle, time (HH:mm, validated on save) and an optional label
• Persist alarms in settings.json via a new AlarmSettings list; legacy single-alarm settings (AlarmEnabled/AlarmTime) are migrated automatically on load

Test plan

• Build passes with 0 warnings / 0 errors
• Legacy settings.json with AlarmEnabled/AlarmTime migrates into the alarm list
• Adding / editing / deleting alarms round-trips through settings.json
• Invalid time formats are rejected with a warning on save
• Alarm fires at the set minute, clock shakes, click stops it; no re-fire within the same minute