Fix Gradle wrapper version mismatch, add distribution checksum

gradle-wrapper.jar was still 9.1.0's wrapper while gradle-wrapper.properties
declared 9.3.1, and the distribution had no distributionSha256Sum to verify
the download against. Flagged by F-Droid's RFP scanner
(https://gitlab.com/fdroid/rfp/-/work_items/4395#note_3827250669); fixed
via the standard `./gradlew wrapper --gradle-version ... --gradle-distribution-sha256-sum ...`
regeneration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy

gradle/wrapper/gradle-wrapper.properties

@@ -1,7 +1,8 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
+distributionSha256Sum=b266d5ff6b90eada6dc3b20cb090e3731302e553a27c5d3e4df1f0d76beaff06
 distributionUrl=https\://services.gradle.org/distributions/gradle-9.3.1-bin.zip
 networkTimeout=10000
 validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME
-zipStorePath=wrapper/dists
+zipStorePath=wrapper/dists