fix: regenerate Android Gradle scaffolding for runnable template (fixes #13)

[valorengels]

Summary

Regenerates the missing Android Gradle scaffolding so the template builds for Android.

Closes #13

Changes

• Add standard Flutter Android platform scaffolding via flutter create --platforms=android: AndroidManifest.xml, build.gradle.kts, settings.gradle.kts, Gradle wrapper, resources.
• Set minSdk = 23 (required by flutter_secure_storage 9.x, per docs/setup_reference.md).
• Upgrade sentry_flutter 8.14.2 -> 9.27.0: required so the generated AGP 9.0.1 / Kotlin 2.3.20 toolchain can compile the plugin (8.14.2 hardcodes Kotlin languageVersion 1.6, dropped in Kotlin 2.x).
• Note in key.properties.example that release signing via key.properties is deferred to the follow-up issue Release signing: key.properties + keystore for make build-android #18.

Verification

• flutter build apk --debug succeeds (exit 0).
• Toolchain (committed): AGP 9.0.1, Kotlin 2.3.20, Gradle wrapper 9.1.0, compileSdk/targetSdk 36, minSdk 23, JDK 17, Android SDK 36.
• applicationId = com.example.flutter_template (derives from pubspec flutter_template; substring match per plan).

Out of scope (per plan)

Release signing (make build-android / flutter build apk --release) is NOT verified here; deferred to follow-up issue #18 "Release signing: key.properties + keystore for make build-android".

[valorengels]

Review: Changes Requested — Tech Debt

The Android Gradle scaffolding is regenerated correctly and the debug build succeeds (verified: flutter build apk --debug → exit 0, app-debug.apk produced). All plan acceptance criteria for the debug-build gate pass: manifest and wrapper present, minSdk = 23, applicationId = com.example.flutter_template (contains flutter_template), existing tracked files preserved, and key.properties.example gained only the planned release-signing NOTE (Risk 2 mitigation). No blockers.

One tech_debt finding: the disclosed release-signing follow-up issue does not exist.

Rubric

• 1. Plan vs. implementation match — pass — debug-build gate fully satisfied; the sentry_flutter upgrade (8.14.2 → 9.27.0) is a documented plan deviation required for the Kotlin 2.x toolchain to compile
• 2. New code quality — pass — standard generated scaffolding, minimal hand edits, clear inline comments
• 3. Test coverage — n/a — build-config-only change; the verification is the debug build, per plan
• 4. Regression risk to existing callers — pass — no Dart/API surface changes; only android/ + pubspec dependency bump
• 5. Data integrity — n/a — no data/schema changes
• 6. Security — pass — no secrets committed; key.properties.example keeps placeholders; keystore ignored
• 7. Documentation accuracy — pass — key.properties.example NOTE accurately describes the deferred signing; README unchanged
• 8. PR body accuracy — pass — build/toolchain claims match reproduced results
• 9. Disclosed deferrals — fail — release-signing deferral claimed but its follow-up issue does not resolve to an open GitHub issue
• 10. Follow-up claims verified — fail — "deferred to follow-up issue Release signing: key.properties + keystore for make build-android" resolves to no open issue

Pre-Verdict Checklist

• 1. All plan acceptance/success criteria validated against diff — PASS — all debug-gate criteria met; release build correctly out of scope
• 2. No-Gos from plan — none violated — PASS — no-Go scope respected (no release signing, no external SDK install)
• 3. New except Exception blocks — each has logger/raise/swallow-ok — N/A — no exception handlers added (GeneratedPluginRegistrant regenerated from template)
• 4. New integration tests — exercise serialization boundary — N/A — no tests added; build-config change
• 5. Plan internal consistency — spike findings match task steps — PASS
• 6. No hardcoded secrets or debug artifacts — PASS — keystore/key.properties properly ignored
• 7. New public APIs — docstrings present — N/A — no new public APIs
• 8. Breaking changes — migration path documented — N/A — no breaking API changes
• 9. Tests added for new behavior — N/A — no runtime behavior added
• 10. Tests cover the failure path — N/A — no new code paths; build is the gate
• 11. UI changes — screenshot captured — N/A — no UI files in diff
• 12. Docs updated for user-facing changes — PASS — key.properties.example NOTE added

Verified

• Code correctness
• Security (no vulnerabilities found)
• Plan requirements met

Blockers

• None

Tech Debt

• android/key.properties.example — release-signing NOTE claims deferral to follow-up issue "Release signing: key.properties + keystore for make build-android" (also in PR body and plan No-Gos/CONCERN 2) — Issue: No such tracking issue exists in the repo (gh issue list --state all shows issues 1–15; searches for "release signing", "key.properties", "keystore" return nothing relevant). The disclosed deferral does not resolve to an open GitHub issue. Severity: tech_debt — Fix: File the concrete follow-up issue (owner: yudame, linked to Make the template a runnable Android project (missing Gradle scaffolding) #13) titled "Release signing: key.properties + keystore for make build-android", and reference its number in the plan's completion note / PR body.

Nits

• Android build output — toolchain advisory — flutter build apk --debug emits a forward-looking warning: "Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): package_info_plus, sentry_flutter. Future versions of Flutter will fail to build if your app uses plugins that apply KGP." This is an upstream plugin-migration advisory, not actionable in this PR, but worth tracking for the template.

Miscellaneous

• None

Acknowledged Deferrals (verified)

• None

Verification Results

• flutter build apk --debug — exit 0 (reproduced; app-debug.apk built)
• test -f android/app/src/main/AndroidManifest.xml — PASS
• test -f android/gradlew — PASS
• grep 'minSdk = 23' android/app/build.gradle.kts — PASS
• grep 'applicationId' android/app/build.gradle.kts — com.example.flutter_template (contains flutter_template)
• git diff --name-status main -- android/ | grep '^D' — no deletions
• Toolchain (committed): AGP 9.0.1, Kotlin 2.3.20, Gradle wrapper 9.1.0, compileSdk/targetSdk 36, minSdk 23, JDK 17 — matches plan decision (a)