build: Lower JDK to 17 and minSdk to 21 (#4369)

.github/copilot-instructions.md

@@ -18,7 +18,7 @@ Meshtastic-Android is a native Android client application for the Meshtastic mes
 **ALWAYS run these commands in the exact order specified to avoid build failures:**
 
 ### Prerequisites Setup
-1. **JDK Requirement:** JDK 21 is required (will fail with other versions)
+1. **JDK Requirement:** JDK 17 is required (compatible with most developer environments)
 2. **Secrets Configuration:** Copy `secrets.defaults.properties` to `local.properties` and update:
    ```properties
    MAPS_API_KEY=your_google_maps_api_key_here
@@ -74,7 +74,7 @@ Meshtastic-Android is a native Android client application for the Meshtastic mes
 ### Environment Setup
 **Required Tools:**
 - Android SDK API 36 (compile target)
-- JDK 21 (NOT JDK 17 or 11 - build will fail)
+- JDK 17 (Preferred for consistency across project and plugins)
 - Gradle 9.0+ (downloaded automatically by wrapper)
 
 **Optional but Recommended:**
@@ -90,16 +90,17 @@ Meshtastic-Android is a native Android client application for the Meshtastic mes
 │   ├── src/androidTest/          # Instrumented tests
 │   ├── src/fdroid/              # F-Droid specific code
 │   └── src/google/              # Google Play specific code
+├── core/                         # Core library modules
 ├── network/                      # HTTP API networking library
 ├── mesh_service_example/         # AIDL service usage example
-├── buildSrc/                     # Build configuration (Configs.kt)
+├── build-logic/                  # Build configuration convention plugins
 └── config/                       # Linting and formatting configs
     ├── detekt/                   # Detekt static analysis rules
     └── spotless/                 # Code formatting configuration
 ```
 
 ### Key Configuration Files
-- `buildSrc/src/main/kotlin/Configs.kt` - Version constants and build config
+- `config.properties` - Version constants and build config
 - `app/build.gradle.kts` - Main app build configuration
 - `config/detekt/detekt.yml` - Static analysis rules
 - `config/spotless/.editorconfig` - Code formatting rules
@@ -140,7 +141,7 @@ Meshtastic-Android is a native Android client application for the Meshtastic mes
 ## Common Issues & Solutions
 
 ### Build Failures
-- **Gradle version error:** Ensure JDK 21 (NOT 17 or 11)
+- **Gradle version error:** Ensure JDK 17 (Compatible version)
 - **Missing secrets:** Copy `secrets.defaults.properties` → `local.properties`
 - **Configuration cache:** Add `--no-configuration-cache` flag if issues persist
 - **Clean state:** Always run `./gradlew clean` before debugging build issues
@@ -153,17 +154,17 @@ Meshtastic-Android is a native Android client application for the Meshtastic mes
 ### Code Style Issues
 - **Formatting:** Run `./gradlew spotlessApply` to auto-fix
 - **Detekt warnings:** Check `config/detekt/detekt.yml` for rules
-- **Localization:** Use `stringResource(R.string.key)` instead of hardcoded strings
+- **Localization:** Use `stringResource(Res.string.key)` instead of hardcoded strings
 
 ## File Organization
 
 ### Source Code Locations
-- **Main Activity:** `app/src/main/java/com/geeksville/mesh/ui/MainActivity.kt`
-- **Service Interface:** `app/src/main/aidl/com/geeksville/mesh/IMeshService.aidl`
-- **UI Screens:** `app/src/main/java/com/geeksville/mesh/ui/`
-- **Data Layer:** `app/src/main/java/com/geeksville/mesh/repository/`
-- **Database:** `app/src/main/java/com/geeksville/mesh/database/`
-- **Models:** `app/src/main/java/com/geeksville/mesh/model/`
+- **Main Activity:** `app/src/main/java/com/geeksville/mesh/MainActivity.kt`
+- **Service Interface:** `core/api/src/main/aidl/org/meshtastic/core/service/IMeshService.aidl`
+- **UI Screens:** `feature/*/src/main/kotlin/org/meshtastic/feature/*/`
+- **Data Layer:** `core/data/src/main/kotlin/org/meshtastic/core/data/`
+- **Database:** `core/database/src/main/kotlin/org/meshtastic/core/database/`
+- **Models:** `core/model/src/main/kotlin/org/meshtastic/core/model/`
 
 ### Dependencies
 - **Non-obvious deps:** Protobuf for device communication, DataDog for analytics (Google flavor)

.github/workflows/merge-queue.yml

@@ -12,8 +12,7 @@ jobs:
   lint:
     if: github.repository == 'meshtastic/Meshtastic-Android'
     uses: ./.github/workflows/reusable-lint.yml
-    secrets:
-      GRADLE_ENCRYPTION_KEY: ${{ secrets.GRADLE_ENCRYPTION_KEY }}
+    secrets: inherit
 
   build:
     needs: lint
@@ -21,11 +20,7 @@ jobs:
     uses: ./.github/workflows/reusable-android-build.yml
     with:
       upload_artifacts: false
-    secrets:
-      GRADLE_ENCRYPTION_KEY: ${{ secrets.GRADLE_ENCRYPTION_KEY }}
-      DATADOG_APPLICATION_ID: ${{ secrets.DATADOG_APPLICATION_ID }}
-      DATADOG_CLIENT_TOKEN: ${{ secrets.DATADOG_CLIENT_TOKEN }}
-      CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+    secrets: inherit
 
   androidTest:
     needs: lint
@@ -35,9 +30,7 @@ jobs:
       api_levels: '[26, 35]' # Run on both API 26 and 35 for merge queue
       test_flavors: 'both' # Run both flavors for merge queue (comprehensive)
       upload_artifacts: false
-    secrets:
-      GRADLE_ENCRYPTION_KEY: ${{ secrets.GRADLE_ENCRYPTION_KEY }}
-      CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+    secrets: inherit
 
   check-workflow-status:
     name: Check Workflow Status  # Matches another in pull-request, and is required for merge to main.

.github/workflows/pull-request.yml

@@ -38,8 +38,7 @@ jobs:
     needs: check-changes
     if: needs.check-changes.outputs.code_changed == 'true'
     uses: ./.github/workflows/reusable-lint.yml
-    secrets:
-      GRADLE_ENCRYPTION_KEY: ${{ secrets.GRADLE_ENCRYPTION_KEY }}
+    secrets: inherit
 
   build:
     needs: lint

.github/workflows/reusable-android-build.yml

@@ -42,10 +42,10 @@ jobs:
           fetch-depth: 0
           submodules: 'recursive'
 
-      - name: Set up JDK 21
+      - name: Set up JDK 17
         uses: actions/setup-java@v5
         with:
-          java-version: '21'
+          java-version: '17'
           distribution: 'jetbrains'
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/reusable-lint.yml

@@ -5,21 +5,25 @@ on:
     secrets:
       GRADLE_ENCRYPTION_KEY:
         required: false
+      GRADLE_CACHE_URL:
+        required: false
 
 jobs:
   lint:
     runs-on: ubuntu-latest  # Lint is fast, doesn't need large runner
     timeout-minutes: 10
+    env:
+      GRADLE_CACHE_URL: ${{ secrets.GRADLE_CACHE_URL }}
     steps:
       - name: Checkout code
         uses: actions/checkout@v6
         with:
           fetch-depth: 0
 
-      - name: Set up JDK 21
+      - name: Set up JDK 17
         uses: actions/setup-java@v5
         with:
-          java-version: '21'
+          java-version: '17'
           distribution: 'jetbrains'
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}