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

README.md

@@ -8,7 +8,7 @@
 
 <div align="center">
   
-[![Android](https://img.shields.io/badge/Android-8.0%2B-3DDC84?style=for-the-badge&logo=android&logoColor=white)](https://www.android.com/)
+[![Android](https://img.shields.io/badge/Android-7.0%2B-3DDC84?style=for-the-badge&logo=android&logoColor=white)](https://www.android.com/)
 [![Kotlin](https://img.shields.io/badge/Kotlin-7F52FF?style=for-the-badge&logo=kotlin&logoColor=white)](https://kotlinlang.org/)
 [![Jetpack Compose](https://img.shields.io/badge/Jetpack%20Compose-4285F4?style=for-the-badge&logo=jetpackcompose&logoColor=white)](https://developer.android.com/compose/)
 [![Material 3](https://img.shields.io/badge/Material_3-6750A4?style=for-the-badge&logo=material-design&logoColor=white)](https://m3.material.io/)
@@ -66,12 +66,14 @@ alt="Get it on F-Droid" align="center" height="80" /></a>
 - 📝 **Offline-first** - Works without internet
 - 📊 **Flexible views** - Switch between list and grid layout
 - ✅ **Checklists** - Tap-to-check, drag & drop
-- 🌍 **Multilingual** - English/German with language selector
 - 🔄 **Configurable sync triggers** - onSave, onResume, WiFi-connect, periodic (15/30/60 min), boot
+- 📌 **Widgets** - Home screen quick-note and note list widgets
+- 🔀 **Smart sorting** - By title, date modified, date created, color
+- ⚡ **Parallel sync** - Downloads up to 5 notes simultaneously
+- 🌍 **Multilingual** - English/German with language selector
 - 🔒 **Self-hosted** - Your data stays with you (WebDAV)
 - 💾 **Local backup** - Export/Import as JSON file (encryption available)
 - 🖥️ **Desktop integration** - Markdown export for Obsidian, VS Code, Typora
-- 🔋 **Battery-friendly** - ~0.2% with defaults, up to ~1.0% with periodic sync
 - 🎨 **Material Design 3** - Dynamic dark/light mode & colors based on system settings
 
 ➡️ **Complete feature list:** [FEATURES.md](docs/FEATURES.md)
@@ -148,6 +150,6 @@ MIT License - see [LICENSE](LICENSE)
 <div align="center">
 <br /><br />
 
-**v1.7.0** · Built with ❤️ using Kotlin + Jetpack Compose + Material Design 3
+**v1.8.1** · Built with ❤️ using Kotlin + Jetpack Compose + Material Design 3
 
 </div>