fix(widgets): keep GlanceAppWidget classes distinct under R8

R8 merged the near-identical widget classes (timer, quick timer,
stopwatch, digital clock) into the analog clock's class. Glance finds a
widget's placed instances by its GlanceAppWidget's class name, so every
updateAll hit every widget: starting a quick timer redrew the timer
widgets' content into the analog and digital clocks until their config
screen re-rendered them. Debug builds were unaffected.

app/proguard-rules.pro

@@ -23,5 +23,11 @@
 # the host, and every widget shows "Can't load widget".
 -keep class * extends androidx.work.InputMerger { <init>(); }
 
+# Glance finds a widget's placed instances by its GlanceAppWidget's class name
+# (`updateAll`, `getGlanceIds`). R8 merged the near-identical widget classes
+# into one, so every widget shared a name: starting a quick timer redrew the
+# timer widgets' content into the analog clock's slots, too.
+-keep class * extends androidx.glance.appwidget.GlanceAppWidget
+
 # Compose Compiler may keep its own; defaults are fine
 -dontwarn org.jetbrains.annotations.**