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.
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