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>

fastlane/metadata/android/en-US/changelogs/20703.txt

@@ -0,0 +1,6 @@
+### Fixed
+- Home-screen widgets no longer get stuck on a loading spinner in the published
+  (F-Droid) release build. They render via Android's background-work system, and
+  release optimisation (R8) was stripping a helper class it loads by name, so the
+  render job never ran. Added the missing keep rule — widgets now load normally.
+