docs: add v1.8.1 changelogs and F-Droid release notes

Changes:
- CHANGELOG.md: Add v1.8.1 section (Bug Fixes, New Features, Improvements, Code Quality)
- CHANGELOG.de.md: Add v1.8.1 section (German translation)
- fastlane/en-US/changelogs/21.txt: F-Droid release notes (EN, 496 chars)
- fastlane/de-DE/changelogs/21.txt: F-Droid release notes (DE, 493 chars)

Changelog covers all 13 branch commits:
- 4 bug fixes (sort persistence, widget scroll, auto-sync toast, drag flicker)
- 4 new features (widget sorting, preview sorting, auto-scroll, cross-boundary drag)
- 3 improvements (sync rate-limiting, toast→banner migration, ProGuard audit)
- 1 code quality (detekt compliance, 0 issues)

F-Droid changelogs stay under 500-char limit.
In-app assets are auto-generated by copyChangelogsToAssets build task.

fastlane/metadata/android/en-US/changelogs/21.txt

@@ -0,0 +1,13 @@
+🛠️ v1.8.1 — CHECKLIST & SYNC FIXES
+
+• Fixed: Sort lost when reopening checklists
+• Fixed: Widget scroll on 3x2 size
+• Fixed: Toast on auto-sync & drag flicker
+• New: Widget checklists with sorting & separator
+• New: Checklist sorting in main preview
+• New: Auto-scroll on line wrap in editor
+• Improved: Sync rate-limiting & battery save
+• Improved: Toasts → unified Banner system
+• Improved: ProGuard for Widgets & Compose
+
+https://github.com/inventory69/simple-notes-sync/blob/main/CHANGELOG.md