Add CI/CD workflows and release signing config

Build workflow: runs tests + assembles debug APK on pull requests.
Release workflow: builds signed APK and uploads to GitHub release.
Signing config reads keystore from env vars (set by CI secrets).
Keystore files excluded from git.

Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>
Co-Authored-By: Claude Opus 4.6 (1M context)

.gitignore

@@ -26,6 +26,10 @@ index.html
 script.js
 styles.css
 
+# Signing
+*.jks
+*.keystore
+
 # Dev artifacts
 PLAN.md
 Screenshot_*.png