chore: bump JVM target from 17 to 21

Metro gradle plugin requires JVM 21+. Update all JDK 17 references:
- app/build.gradle.kts: sourceCompatibility, targetCompatibility, jvmTarget
- .devcontainer: base image java:1-17-bookworm → java:1-21-bookworm
- docs: .github/copilot-instructions.md and CLAUDE.md

.devcontainer/README.md

@@ -4,7 +4,7 @@ This directory contains the development container configuration for TRMNL Androi
 
 ## Features
 
-- **Base Image**: Java 17 (Bookworm)
+- **Base Image**: Java 21 (Bookworm)
 - **Android SDK**: Manually installed via post-create script
 - **Android Command Line Tools**: Version 11076708
 - **Platform Tools**: Latest version

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
   "name": "TRMNL Android",
-  "image": "mcr.microsoft.com/devcontainers/java:1-17-bookworm",
+  "image": "mcr.microsoft.com/devcontainers/java:1-21-bookworm",
 
   "features": {
     "ghcr.io/devcontainers/features/git:1": {},

.github/copilot-instructions.md

@@ -55,7 +55,7 @@ This document provides essential information for GitHub Copilot agents working o
 
 ## Environment Requirements
 
-- **JDK Version:** 17 (OpenJDK 17 - Temurin distribution recommended)
+- **JDK Version:** 21 (OpenJDK 21 - Temurin distribution recommended)
 - **Gradle:** 8.13 (via wrapper, do NOT install manually)
 - **Android SDK:** Compile SDK 36
 - **Build Tools:** Managed by Gradle plugin (AGP 8.9.2)
@@ -199,7 +199,7 @@ See `RELEASE_CHECKLIST.md` for complete release process.
 ## Trust These Instructions
 
 These instructions have been validated by running all commands in a clean repository clone. If you encounter issues not documented here:
-1. First verify you're using JDK 17 and the Gradle wrapper (`./gradlew`)
+1. First verify you're using JDK 21 and the Gradle wrapper (`./gradlew`)
 2. Try `./gradlew clean` then retry the command
 3. Check if similar issues exist in closed GitHub issues
 4. Only search the codebase if these instructions prove incomplete or incorrect

CLAUDE.md

@@ -53,7 +53,7 @@ TRMNL Android is a native Android app that displays TRMNL e-ink device content o
 ```
 
 **Notes:**
-- JDK: Minimum JDK 17, but JDK 21+ works (backward compatible)
+- JDK: Minimum JDK 21 (required by Metro gradle plugin)
 - ALWAYS use `./gradlew` wrapper, NEVER system gradle
 - First build: ~1-2 minutes (optimized with G1GC, parallel execution, configuration cache)
 - Subsequent builds: ~2-8 seconds with Gradle cache and configuration cache

app/build.gradle.kts

@@ -98,8 +98,8 @@ android {
     }
 
     compileOptions {
-        sourceCompatibility = JavaVersion.VERSION_17
-        targetCompatibility = JavaVersion.VERSION_17
+        sourceCompatibility = JavaVersion.VERSION_21
+        targetCompatibility = JavaVersion.VERSION_21
     }
 
     buildFeatures {
@@ -119,7 +119,7 @@ android {
 
 kotlin {
     compilerOptions {
-        jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17)
+        jvmTarget.set(org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_21)
     }
 }