Upgrade build tooling: Gradle 9.2.1→9.6.1 + AGP 8.12.3→9.x (major)

[almothafar]

Tracking issue — deferred (do after the routine dependency bumps). Filed now so it's in the backlog; not yet scheduled.

Scope

• Gradle wrapper: 9.2.1 → 9.6.1 (gradle/wrapper/gradle-wrapper.properties)
• AGP com.android.application: 8.12.3 → 9.1.1 (build.gradle root) — major version jump (8 → 9)

Why it needs care

AGP 9 is a major release with breaking changes: removes deprecated DSL/APIs, changes some defaults, requires Gradle 9.x (we have it) and JDK 17+ (we're on 25). This app is Java-only (no Kotlin), so the migration is usually smooth, but:

• Must build clean (assembleDebug + assembleRelease) — the release build uses R8/minify, worth confirming.
• Requires an on-device retest since AGP can shift build/packaging behavior.
• Consider running the AGP Upgrade Assistant / reviewing the AGP 9 release notes for any DSL our build.gradle uses (e.g. the incrementVersion task hook, lint { error 'MissingTranslation' }).

Plan when scheduled

1. Bump Gradle wrapper → 9.6.1, verify build.
2. Bump AGP → 9.1.1, fix any DSL breakages, verify assembleDebug + assembleRelease + test.
3. Build to device, user retests core flows.

[almothafar]

Note from #80/#82: androidx.core:core is capped at 1.18.0 for now because core 1.19.0 hard-requires AGP 9.1.0+. Bumping core to 1.19.0 should be part of this AGP 9 upgrade. Other latest app libs (material 1.14.0, activity 1.13.0) were fine on AGP 8.12.3 and already shipped in #82.

[almothafar]

Parked (2026-07-04). Decided not to pursue AGP 9 for now: this is a small Java-only app that builds fine on AGP 8.x, and AGP 9's headline change (built-in Kotlin) is a net negative here — it forced a kotlin-gradle-plugin pin just to keep Java 25, plus a compileSdk 37 bump for core:1.19.0 (which the app doesn't need). Revisit when something concretely requires it (a dependency that needs API 37 / AGP 9, or AGP 8.x nearing EOL). PR #85 (closed) holds the working diff. Proceeding separately with just Gradle 9.6.1 + deprecation cleanup on AGP 8.