Add copilot support

.github/copilot-instructions.md

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+We use Java 21 (eclipse-temurin:21-jdk-jammy) to build the app via gradle.
+
+Only use the "debug" flavor of the app when you make builds.
+You cannot build "release" as the signing keys are not part of the repository.
+
+Do not try to upgrade the kotlin version in "gradle/libs.versions.toml", it will throw a lot of warnings and errors.

.github/copilot/environment.txt

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+https://github.com/Catfriend1/syncthing-android/settings/environments
+    copilot
+        COPILOT_AGENT_FIREWALL_ALLOW_LIST_ADDITIONS
+            dl.google.com,maven.org,jitpack.io,repo.gradle.org,services.gradle.org

.github/workflows/copilot-setup-steps.yml

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+name: "Copilot Setup Steps"
+
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+
+    permissions:
+      contents: read
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Java 21
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '21'
+
+      - name: Cache Gradle packages
+        uses: actions/cache@v3
+        with:
+          path: ~/.gradle/caches
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-