docs(v1.8.1): comprehensive metadata & documentation update for F-Droid release

Complete overhaul of all metadata and documentation for v1.8.1:

F-Droid Critical:
- Fix broken umlauts in de-DE/10.txt changelog (für, hinzugefügt, aufgeräumt)
- Shorten de-DE/7.txt changelog for brevity
- Remove trailing newline from en-US/title.txt

Version & Accuracy:
- Update README badges (Android 7.0+) and footer (v1.8.1)
- Add widgets, sorting, parallel sync to highlights
- Fix APK names in workflows (removed "universal")
- Update cache action v3 → v4 in PR workflow
- Fix CONTRIBUTING.md filename references
- Update QUICKSTART guides (APK name, typo, Android version, docs links)

Feature Documentation:
- Update full_description.txt (both locales) - remove NEW/NEU labels, add v1.8.0+ features
- Major FEATURES.md update - add Views & Layout, Widgets sections, updated tech stack
- Add UPCOMING.md v1.7.0-1.8.1 as released sections
- Update DOCS.md roadmap references and dates to Feb 2026
- Create missing en-US changelogs 1.txt, 2.txt

Supplementary Fixes:
- Update BACKUP.md - add encryption docs, fix cross-references
- Add CHANGELOG.md reference links v1.2.1-v1.8.1 (15 missing)
- Fix DEBUG_APK.md duplicate header
- Rewrite fastlane/README.md with both locales and limits table
- Create SELF_SIGNED_SSL.de.md (full German translation)

Affects: 26 files across READMEs, docs/, fastlane/, workflows

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

@@ -1,29 +1,31 @@
 Simple Notes Sync is a minimalist note-taking app with WebDAV sync and modern features.
 
 Key Features:
-• Text notes and checklists (tap-to-check, drag & drop)
-• NEW: Grid view for notes
+• Text notes and checklists (tap-to-check, drag & drop, sorting)
+• Grid and list view with note color support
+• Homescreen widgets (quick-note, checklist with interactive checkboxes)
 • Multi-device sync (phone, tablet, desktop)
 • WebDAV sync with your own server (Nextcloud, ownCloud, etc.)
 • Markdown export/import for desktop editors (Obsidian, VS Code)
-• NEW: WiFi-only sync, VPN support, encryption for local backups
+• WiFi-only sync, VPN support, parallel downloads
 • Configurable sync triggers: onSave, onResume, WiFi, periodic, boot
-• Fully usable offline
-• No ads, no trackers
+• Fully usable offline – no ads, no trackers
 
 Privacy & Security:
 • Your data stays with you – no cloud, no tracking libraries
 • Support for self-signed SSL certificates
-• SHA-256 hash of signing certificate shown in app and releases
+• Encrypted local backups
 
 Synchronization:
-• Automatic or manual, optimized performance, optional periodic sync
-• Smart conflict resolution, deletion tracking, batch actions
+• Parallel downloads (up to 5 simultaneous)
+• Live sync progress with phase indicators
+• Smart conflict resolution, server deletion detection
+• Post-update changelog dialog
 
 UI & Design:
 • Modern Jetpack Compose interface
 • Material Design 3, dynamic colors, dark mode
-• Animations and live sync status
+• Note & checklist sorting (title, date, color, alphabetical)
 
 Multilingual:
 • English and German, automatic detection, in-app language selector