v2.0 - Big quality-of-life update.

* "Add widget" button right inside the app.
* Reconfigure widgets after adding (pencil icon while moving).
* New settings: tracked amount, decimals, thousands separator,
  background opacity, hide-logo, hide-caption.
* "Bitcoin (₿)" easter-egg currency that always shows 1 BTC.
* Cached price stays visible (greyed) on any fetch failure.
* Battery-saver awareness: greys out, taps explain, deep-links
  into battery settings.
* Offline detection: greys out instead of retrying.
* Errors now reported in the main app, not in notifications —
  POST_NOTIFICATIONS permission removed.
* New Bitcoin ring logo; significantly smaller APK.
