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

gradle/wrapper/gradle-wrapper.properties

@@ -0,0 +1,5 @@
+distributionBase=GRADLE_USER_HOME
+distributionPath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-9.7.1-bin.zip
+zipStoreBase=GRADLE_USER_HOME
+zipStorePath=wrapper/dists