Add showcase release workflow and gitignore build artifacts

- Remove 541MB of committed Electron build artifacts from git
- Add .gitignore patterns for release/, *.apk, *.aab, *.ipa
- Add GitHub Actions workflow for building showcase releases
  (manual dispatch with version input, builds Linux desktop +
  Android APK, creates GitHub Release with attached artifacts)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,10 @@ Thumbs.db
 # Logs
 *.log
 npm-debug.log*
+
+# Showcase build artifacts
+release/
+*.apk
+*.aab
+*.ipa
+*.xcarchive