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

docs/BACKUP.md

@@ -276,8 +276,9 @@ Step-by-step:
 
 ### Data Protection
 - ✅ **Locally stored** - No cloud upload without your action
-- ✅ **No encryption** - Plain text format for readability
-- ⚠️ **Sensitive data?** - Encrypt backup file yourself (e.g., 7-Zip with password)
+- ✅ **Optional encryption** _(v1.7.0+)_ - Password-protect backup files
+- ✅ **Human-readable** - Plain JSON format when unencrypted
+- ⚠️ **Sensitive data?** - Enable encryption or use external tools (e.g., 7-Zip)
 
 ### Recommendations
 - 🔐 Store backup files in encrypted container
@@ -317,8 +318,8 @@ Step-by-step:
 ---
 
 **📚 See also:**
-- [QUICKSTART.en.md](../QUICKSTART.en.md) - App installation and setup
-- [FEATURES.en.md](FEATURES.en.md) - Complete feature list
-- [DESKTOP.en.md](DESKTOP.en.md) - Desktop integration with Markdown
+- [QUICKSTART.md](../QUICKSTART.md) - App installation and setup
+- [FEATURES.md](FEATURES.md) - Complete feature list
+- [DESKTOP.md](DESKTOP.md) - Desktop integration with Markdown
 
-**Last update:** v1.2.1 (2026-01-05)
+**Last update:** v1.8.1 (2026-02-11)