Add rate projection coloring, live countdown, and reset notification

[ramonsmits]

Summary

Three features that improve usage monitoring:

• Rate projection coloring — bars now turn yellow/red based on projected utilization at current consumption rate, not just current percentage. If you're at 30% but only 1h into a 5h window, that projects to 150% — red. Previously this would show green.
• Live countdown timer — "Resets in" labels now update continuously (every minute, switching to every second in the final minute) instead of only on API poll
• Reset notification with sound — when a window was at 100% and resets, plays a racing-start beep sequence (beep-beep-beep-beeeep) and shows a GNOME notification

Details

Rate projection

• projected = (utilization / elapsed) * total_window
• Skips projection when <5% of window has elapsed (not enough data)
• Applied to both panel bar and dropdown bars
• Dropdown shows "(proj: 95%)" when warning/critical

Live countdown

• Stores resets_at from API in instance variables
• _scheduleCountdownTick() picks the right interval: 60s normally, 1s in final minute
• Shows seconds in the final minute (e.g. "42s", "2m 15s")

Reset notification

• Tracks _prevWasLimited state per window
• Detects when utilization drops from >=100% to <80% (reset occurred)
• Also triggers on countdown reaching zero if window was limited
• Sound: 3x 880Hz short beeps + 1x 1320Hz long beep via gst-launch-1.0

Cleanup

• All new timers properly cleaned up in destroy()
• Reset-refresh timeout tracked and cancelled on destroy

Test plan

• Verify bars show projection-based colors (not raw thresholds)
• Verify countdown labels update in real-time
• Verify seconds shown in final minute of countdown
• Verify notification + sound plays on reset (can test by restarting with mock data)
• Verify clean disable/enable cycle (no timer leaks)