Fresh app icon

* Tryst has a new home-screen icon: a T-heart mark on a pink-to-purple gradient. Replaces the placeholder flame that had been shipping since v0.2.
* Adaptive-icon setup so launcher shape masks (circle, squircle, teardrop) render cleanly on Android 12+.
* No feature or behaviour changes in this release — the app under the icon is v0.5.7 with the same features.
