chore(fdroid): single-source fastlane metadata + reproducible-build prep

Make fastlane/metadata/android/ the single source of truth for store
listing metadata, consumed directly by the official F-Droid repo and
transformed into the self-hosted repo's localized layout at release time.

- Move fdroid-metadata/<appid>/<locale>/ -> fastlane/metadata/android/<locale>/
  (git-tracked renames: summary->short_description, description->full_description,
  + title.txt, images/icon.png, images/phoneScreenshots/); keep the app-level
  .yml control file for the self-hosted `fdroid update`.
- Add scripts/fastlane_to_fdroid_localized.sh (fastlane -> F-Droid localized,
  incl. changelogs) and scripts/sync_changelog_to_fastlane.sh (CHANGELOG.md ->
  fastlane changelog); verified byte-identical to the previous metadata.
- release.yaml: build self-hosted metadata from the fastlane tree and sync the
  per-version changelog before the transform (one changelog source for both
  channels).
- Disable AGP VCS-info embedding on release builds (vcsInfo { include = false })
  so builds reproduce byte-for-byte vs the distributed APK — the only file that
  otherwise differed (META-INF/version-control-info.textproto). Effective from
  the next release.
- Add docs/fdroid-official/ (draft fdroiddata recipe: reproducible build +
  AllowedAPKSigningKeys + Binaries + notes).
- Repoint README screenshots/icon, update docs/README + RELEASING, and skip the
  Android build on fastlane-only changes (ci.yaml).

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

fastlane/metadata/android/en-US/changelogs/20701.txt

@@ -0,0 +1,8 @@
+### Fixed
+- Fixed the app crashing immediately on launch whenever calendar access hadn't
+  been granted yet (a fresh install, or after revoking the permission). The app
+  set up its live calendar-change listener before the permission screen could
+  appear, which newer Android versions reject outright — so the app died before
+  you could grant access. The listener now waits for the permission and attaches
+  itself the moment it's granted.
+

fastlane/metadata/android/en-US/full_description.txt

@@ -0,0 +1,15 @@
+Calendula is a modern, open-source calendar app for Android. It works
+directly on the system calendar provider, so any source synced to your
+device — Nextcloud via DAVx5, Google, local, WebCal subscriptions — shows up
+automatically, and changes you make sync back the same way.
+
+Create, edit and delete events, including recurring events with scoped
+changes (only this event / this and all following / the whole series) and a
+simple repeat picker. Calendula also delivers your event reminders as
+notifications — tap one and you're on the event.
+
+The differentiator is the design: real Material 3 Expressive throughout,
+with dynamic color, expressive motion, and expressive shapes.
+
+Privacy: zero telemetry, no analytics, no network access — your data never
+leaves the device.

fastlane/metadata/android/en-US/short_description.txt

@@ -0,0 +1 @@
+A modern Material 3 Expressive calendar for Android.

fastlane/metadata/android/en-US/title.txt

@@ -0,0 +1 @@
+Calendula