v0.2.0: always-visible icon, live percentage badge, faster refresh, single-instance guard, extra-usage line, threshold alerts

- Self-promote the tray icon out of the overflow chevron via the
  NotifyIconSettings registry entry, so it's actually always visible.
- Render the higher of the two utilization percentages directly on the
  icon (no hover needed) instead of just a color.
- Poll every 20s (was 60s) and also refresh on tray hover/click
  (debounced), addressing stale numbers requiring manual refresh.
- Guard against duplicate instances with a named Win32 mutex.
- Optional third menu line for accounts with pay-as-you-go extra usage
  enabled, parsed leniently so it can't break the core two percentages.
- One-time Windows balloon notification when either window crosses 90%.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>