Add optional remaining-time countdown to menu bar title

[DaniSchenk]

What

Adds a "Show remaining time in window" menu toggle (persisted in config, off by default) that appends a compact reset countdown to each window's percentage in the menu bar title:

5h 37% (1h12m) · 7d 10% (3d4h)

Why

The percentages alone don't tell you when a window frees up. This surfaces the reset countdown inline for anyone pacing work against the 5h / 7d limits, without cluttering the bar for people who don't want it.

How it works

• Config: new show_remaining_time field (defaults to false), toggled from the tray menu and saved to config.json.
• Rendering: short_countdown renders finer than the menu's existing reset_suffix — minute precision above 2h, 2-unit max (3d4h, 1h12m, 45m). It returns None past the deadline so a stale percentage never sits next to a countdown implying the window is still blocked.
• Refresh cadence: the countdown is time-driven, not data-driven — polls are 15–30 min apart, so nothing would otherwise repaint as the minute rolls. A dedicated once-a-minute MinuteTick event (mirroring the existing BlinkTick ticker pattern) repaints the title. The handler short-circuits unless the feature is on and the alarm isn't blinking. Cost is negligible: the native set_title already fingerprints segments and skips the AppKit call when nothing changed, so an unchanged repaint just rebuilds a small Vec once a minute.

Notes for reviewers

• Single-file change (src/bin/menubar.rs); no new dependencies.
• Feature is opt-in and off by default — no behavior change unless the user enables it.
• Unit tests cover the countdown bucketing edges (short_countdown_buckets): day/hour/minute boundaries, sub-minute floor to 1m, expired → None.

🤖 Generated with Claude Code

resolves #1