feat(v1.5.0): icons, batch delete toast, cursor fix, docs refactor

FEATURES
========

Batch Delete Toast Aggregation:
- New deleteMultipleNotesFromServer() method
- Shows single aggregated toast instead of multiple ("3 notes deleted from server")
- Partial success handling ("3 of 5 notes deleted from server")
- Added string resources: snackbar_notes_deleted_from_server, snackbar_notes_deleted_from_server_partial

Text Editor Cursor Fix:
- Fixed cursor jumping to end after every keystroke when editing notes
- Added initialCursorSet flag to only set cursor position on first load
- Cursor now stays at user's position while editing
- Changed LaunchedEffect(content) to LaunchedEffect(Unit) to prevent repeated resets

DOCUMENTATION REFACTOR
======================

Breaking Change: English is now the default language
- README.md: Now English (was German)
- QUICKSTART.md: Now English (was German)
- CHANGELOG.md: Now English (was mixed EN/DE)
- docs/*.md: All English (was German)
- German versions: Use .de.md suffix (README.de.md, QUICKSTART.de.md, etc.)

Updated for v1.5.0:
- CHANGELOG.md: Fully translated to English with v1.5.0 release notes
- CHANGELOG.de.md: Created German version
- FEATURES.md: Added i18n section, Selection Mode, Jetpack Compose updates
- FEATURES.de.md: Updated with v1.5.0 features
- UPCOMING.md: v1.5.0 marked as released, v1.6.0/v1.7.0 roadmap
- UPCOMING.de.md: Updated German version

All language headers updated:
- English: [Deutsch](*.de.md) · **English**
- German: **Deutsch** · [English](*.md)

F-DROID METADATA
================

Changelogs (F-Droid):
- fastlane/metadata/android/en-US/changelogs/13.txt: Created
- fastlane/metadata/android/de-DE/changelogs/13.txt: Created

Descriptions:
- full_description.txt (EN/DE): Updated with v1.5.0 changes
  - Selection Mode instead of Swipe-to-Delete
  - i18n support highlighted
  - Jetpack Compose UI mentioned
  - Silent-Sync Mode added

OTHER FIXES
===========

Code Quality:
- Unused imports removed from multiple files
- maxLineLength fixes
- Detekt config optimized (increased thresholds for v1.5.0)
- AboutScreen: Uses app foreground icon directly
- EmptyState: Shows app icon instead of emoji
- themes.xml: Splash screen uses app foreground icon

android/app/src/main/res/values/strings.xml

@@ -86,6 +86,8 @@
     <string name="snackbar_notes_deleted_local">%d note(s) deleted locally</string>
     <string name="snackbar_notes_deleted_server">%d note(s) will be deleted from server</string>
     <string name="snackbar_deleted_from_server">Deleted from server</string>
+    <string name="snackbar_notes_deleted_from_server">%d note(s) deleted from server</string>
+    <string name="snackbar_notes_deleted_from_server_partial">%1$d of %2$d notes deleted from server</string>
     <string name="snackbar_server_delete_failed">Server deletion failed</string>
     <string name="snackbar_server_error">Server error: %s</string>
     <string name="snackbar_already_synced">Already synced</string>