Add Pomodoro timer to dashboard

[Mentigen]

Summary

Adds a Pomodoro timer as a new tab in the dashboard.

• Circular progress ring with drag-to-seek support
• Work / short break / long break phases with a dot strip indicator
• Play/pause, skip, and reset controls
• Idle mode tracking after a break ends
• Settings section: configurable durations, long break toggle, sound notifications with volume control
• Persistent cycle and pomodoro counters

Test plan

• Open dashboard and switch to the Pomodoro tab
• Start, pause, and skip phases — ring and dot strip update correctly
• Drag the ring to seek within the current phase
• Open Settings, adjust durations via SpinBox (keyboard input works)
• Toggle long break and sound, verify behaviour
• Let a phase complete — sound plays, idle mode activates
• Verify Media and other tabs are unaffected

[sufficientpast]

this fits more likely to be a plugin imo #1384

[Mentigen]

Thanks for the pointer to #1384! I agree that a plugin system would be the ideal home for niche features like this. However, #1384 is still a feature request with no implementation yet - the dashboard tabs injection point listed there doesn't exist in code. There's no plugin mechanism to use right now.

In the meantime, this PR follows the exact same pattern as existing dashboard tabs, so the integration is minimal and self-contained. If the plugin system lands in the future, the Pomodoro module could be extracted into a standalone plugin without touching core.

Happy to hear the maintainers' view on whether to hold off or proceed!