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/changelogs/2.txt

@@ -0,0 +1,5 @@
+• Configurable sync interval (15/30/60 minutes)
+• Transparent battery usage display (measured: 0.4%/day at 30min)
+• Doze Mode optimizations for more reliable background syncs
+• About section with app information and GitHub links
+• Various bugfixes and performance improvements