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/build.gradle.kts

@@ -28,8 +28,8 @@ android {
         // which builds this version and then creates the matching vX.Y.Z tag +
         // release itself (versionCode is pinned to MAJOR*10000 + MINOR*100 +
         // PATCH from versionName, e.g. 2.7.2 -> 20702). See docs/RELEASING.md.
-        versionCode = 20702
-        versionName = "2.7.2"
+        versionCode = 20703
+        versionName = "2.7.3"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
     }