feat: add pomodoro timer widget

[bottlebrushes]

Summary

• Adds a new default.pomodoro widget with a compact menu bar timer and popup controls
• Supports work/break/long break phases with automatic transitions and session tracking
• Popup includes circular progress ring, play/pause/skip/stop controls, and a settings panel to adjust durations
• Sends macOS notifications when phases end

Test plan

• Add "default.pomodoro" to the displayed array in ~/.barik-config.toml
• Verify the timer icon appears in the menu bar
• Click to open popup, start a timer, verify countdown works
• Test pause/resume/skip/stop controls
• Open settings (gear icon) and adjust durations
• Verify phase transitions (work -> break -> work, long break after N sessions)

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