Replace background and sound effects with own artwork

The Pexels forest photo is replaced by the author's own background, so
its credit is dropped from the Licenses screen (all six locales) and the
NonFreeAssets anti-feature is removed from the F-Droid recipe. Sound
effects are replaced as well, and the old generator script in tools/ is
removed. Settings no longer duplicates the opening-bar switch, which
lives in Appearance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015g6ECVSKcBG8CJPrqC3G5o

README.md

@@ -87,7 +87,6 @@ Nearby multiplayer needs two physical devices — the emulator has no real Bluet
 ├── fastlane/            # F-Droid store listing, screenshots and changelogs
 ├── fdroid/              # Build recipe for fdroiddata
 ├── gradle/              # Gradle wrapper
-├── tools/               # Helper scripts (sound effect generation)
 ├── CHANGELOG.md         # Per-version build dates, sizes and checksums (links to GitHub Releases)
 ├── LICENSE              # GNU GPLv3
 └── README.md