Add app source for F-Droid release (v1.0.43)

Adds the full Android source tree with unit tests, fastlane store
metadata and the Gradle wrapper. The archive shipped without
gradle-wrapper.jar, so the official Gradle 9.7.1 wrapper jar is
included to make ./gradlew work from a fresh clone. .gitignore now
also excludes build output and keystores. README gains build
instructions and an updated structure tree.

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

.gitignore

@@ -1,10 +1,24 @@
+# Gradle / Android Studio
+.gradle/
+build/
+local.properties
+.idea/
+*.iml
+.kotlin/
+captures/
+.externalNativeBuild/
+.cxx/
+
+# Signing keys - never commit these
+*.jks
+*.keystore
+
 # OS
 .DS_Store
 Thumbs.db
 
 # Editors
 .vscode/
-.idea/
 *.swp
 
 # Local scratch