fix(widget): keep WorkManager InputMerger so widgets render under R8 (#18)

Home-screen widgets were stuck on the Glance loading layout (a blank spinner)
in the minified release build — but worked in debug. Root cause: Glance renders
every widget through a WorkManager worker (androidx.glance.session.SessionWorker),
and WorkManager instantiates the InputMerger reflectively from the fully-qualified
class name persisted in the WorkSpec (Class.newInstance, no-arg ctor). Under R8
full mode (AGP 9 default) the unused no-arg constructor of
androidx.work.OverwritingInputMerger was stripped, so WorkManager threw
"OverwritingInputMerger has no zero argument constructor", the SessionWorker
never ran, and provideContent never executed — leaving the widget on its
initial loading layout forever.

Same R8-reflection family as the v2.7.0 Room keep-rule fix. Keep the name +
constructor of every androidx.work.InputMerger.

Verified on-device with the releaseTest build (R8-minified): the agenda widget,
which showed only a spinner before, now renders its content; the WM-InputMerger
InstantiationException is gone. Closes #18. Cuts v2.7.3.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/proguard-rules.pro

@@ -15,3 +15,14 @@
 # Keep the generated Room database implementations fully intact.
 -keep class * extends androidx.room.RoomDatabase { *; }
 -dontwarn androidx.room.paging.**
+
+# WorkManager instantiates an InputMerger reflectively (Class.newInstance) from
+# the fully-qualified class name persisted in the WorkSpec, so the class must
+# keep both its name and a no-arg constructor. Glance renders every widget
+# through a WorkManager worker (androidx.glance.session.SessionWorker) whose
+# default merger is androidx.work.OverwritingInputMerger. Under R8 full mode
+# (AGP 9 default) that unused no-arg constructor was stripped, so WorkManager
+# threw "OverwritingInputMerger has no zero argument constructor", the
+# SessionWorker never ran, and widgets were stuck on their loading layout
+# (a blank spinner) in release builds. Keep every InputMerger's name + ctor.
+-keep class * extends androidx.work.InputMerger { <init>(...); }