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

CONTRIBUTING.md

@@ -94,10 +94,10 @@ Nutze die [Feature Request Template](https://github.com/inventory69/simple-notes
 Dokumentations-Verbesserungen sind auch Contributions! 
 
 **Dateien:**
-- `README.md` / `README.en.md` - Übersicht
-- `QUICKSTART.md` / `QUICKSTART.en.md` - Schritt-für-Schritt Anleitung
-- `DOCS.md` / `DOCS.en.md` - Technische Details
-- `server/README.md` / `server/README.en.md` - Server Setup
+- `README.de.md` / `README.md` - Übersicht
+- `QUICKSTART.de.md` / `QUICKSTART.md` - Schritt-für-Schritt Anleitung
+- `docs/DOCS.de.md` / `docs/DOCS.md` - Technische Details
+- `server/README.de.md` / `server/README.md` - Server Setup
 
 **Bitte:** Halte beide Sprachen (DE/EN) synchron!
 
@@ -219,10 +219,10 @@ Use the [Feature Request Template](https://github.com/inventory69/simple-notes-s
 Documentation improvements are also contributions!
 
 **Files:**
-- `README.md` / `README.en.md` - Overview
-- `QUICKSTART.md` / `QUICKSTART.en.md` - Step-by-step guide
-- `DOCS.md` / `DOCS.en.md` - Technical details
-- `server/README.md` / `server/README.en.md` - Server setup
+- `README.de.md` / `README.md` - Overview
+- `QUICKSTART.de.md` / `QUICKSTART.md` - Step-by-step guide
+- `docs/DOCS.de.md` / `docs/DOCS.md` - Technical details
+- `server/README.de.md` / `server/README.md` - Server setup
 
 **Please:** Keep both languages (DE/EN) in sync!
 
@@ -260,4 +260,4 @@ By contributing, you agree that your code will be published under the [MIT Licen
 
 Öffne ein [Issue](https://github.com/inventory69/simple-notes-sync/issues) oder nutze die [Question Template](https://github.com/inventory69/simple-notes-sync/issues/new/choose).
 
-**Frohe Weihnachten & Happy Coding! 🎄**
+**Happy Coding! 🚀**