v0.3.0: legible icon, fix 429 rate-limit storm, floating panel, configurable poll interval

- Bold, outlined percentage digits on the tray icon (verified by rendering
  and downscaling real previews to 16/24/32px) -- the old thin font turned
  to mush at real tray-icon size.
- Fix a real production bug: 20-30s polling was hitting the usage endpoint's
  own rate limit (HTTP 429), which made the widget show "unavailable" more
  than before this app existed. Failures now back off exponentially and
  respect Retry-After (with a floor, since the live endpoint sends
  Retry-After: 0 while actively rate-limiting).
- New optional floating usage panel (tray menu > Usage panel): real drawn
  progress bars, Both/5h/Weekly/Rotating modes, remembered across restarts.
- New configurable poll interval (tray menu > Poll interval): 1/2/5/10
  minutes, 1 minute is both default and a hard floor -- no need to spam
  Anthropic.
- Accurate unavailable messaging: distinguishes "sign-in needed" from
  "rate-limited, retrying" and "network error, retrying" instead of one
  generic (and actively misleading during a 429) message.

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