Finalize 4.1.4 release prep

- BUILD_IMAGE.md: Clarify docs are for devs, not end users
- Add 4.1.5 plan with decode progress bar feature
- Update .gitignore for release assets (.zip, .img)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -86,6 +86,10 @@ pishrink.sh
 frontends/web/temp_files/
 rpi/config.json
 
-# Pre-built Pi tarballs (too large for git)
+# Pre-built Pi tarballs and images (release assets, too large for git)
 rpi/stegasoo-pi-arm64.tar.zst
+rpi/stegasoo-pi-arm64.tar.zst.zip
 rpi/stegasoo-venv-pi-arm64.tar.zst
+rpi/*.img
+rpi/*.img.zst
+rpi/*.img.zst.zip