feat: Add Linux desktop build support to CI/CD workflow

Add build-linux job to compile Linux x64 desktop application and
integrate it into the R2 upload and release workflows.

Changes:
- New build-linux job: Builds Flutter Linux desktop app with GTK-3
- Archives Linux build as tar.gz for distribution
- Added Linux artifact download in upload-to-r2 job
- Updated artifact preparation to copy Linux archives
- Added Linux download card (🐧) to generated index.html
- Included Linux artifacts in GitHub releases
- Added Linux to download summary in workflow output

The Linux build produces MeshCore-SAR-Linux.tar.gz containing the
compiled application bundle ready for x64 Linux systems.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/build-multiplatform.yml

@@ -448,12 +448,67 @@ jobs:
           path: MeshCore-SAR-Windows.zip
           retention-days: 30
 
+  # Linux Build
+  build-linux:
+    name: Build Linux AppImage
+    runs-on: ubuntu-latest
+    needs: [update-version]
+    if: always() && (needs.update-version.result == 'success' || needs.update-version.result == 'skipped')
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Download versioned pubspec
+        if: startsWith(github.ref, 'refs/tags/v')
+        uses: actions/download-artifact@v4
+        with:
+          name: versioned-pubspec
+          path: .
+
+      - name: Setup Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          flutter-version: ${{ env.FLUTTER_VERSION }}
+          channel: 'stable'
+          cache: true
+
+      - name: Install Linux dependencies
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install -y clang cmake ninja-build pkg-config libgtk-3-dev liblzma-dev libstdc++-12-dev
+
+      - name: Enable Linux desktop
+        run: flutter config --enable-linux-desktop
+
+      - name: Install dependencies
+        run: flutter pub get
+
+      - name: Generate localizations
+        run: flutter gen-l10n
+
+      - name: Build Linux app
+        run: flutter build linux --release
+
+      - name: Archive Linux Build
+        run: |
+          cd build/linux/x64/release/bundle
+          tar -czf ../../../../../MeshCore-SAR-Linux.tar.gz *
+          cd ../../../../../
+
+      - name: Upload Linux Build
+        uses: actions/upload-artifact@v4
+        with:
+          name: linux-executable
+          path: MeshCore-SAR-Linux.tar.gz
+          retention-days: 30
+
   # Upload to Cloudflare R2 (Unstable Bucket)
   upload-to-r2:
     name: Upload to Cloudflare R2
-    needs: [build-android, build-ios, build-macos, build-windows]
+    needs: [build-android, build-ios, build-macos, build-windows, build-linux]
     runs-on: ubuntu-latest
-    if: always() && (needs.build-android.result == 'success' || needs.build-ios.result == 'success' || needs.build-macos.result == 'success' || needs.build-windows.result == 'success')
+    if: always() && (needs.build-android.result == 'success' || needs.build-ios.result == 'success' || needs.build-macos.result == 'success' || needs.build-windows.result == 'success' || needs.build-linux.result == 'success')
 
     steps:
       - name: Checkout code for git log
@@ -493,6 +548,14 @@ jobs:
           name: windows-executable
           path: artifacts/windows-executable
 
+      - name: Download Linux Executable
+        if: needs.build-linux.result == 'success'
+        uses: actions/download-artifact@v4
+        continue-on-error: true
+        with:
+          name: linux-executable
+          path: artifacts/linux-executable
+
       - name: Verify downloaded artifacts
         run: |
           echo "=== Downloaded build artifacts for R2 upload ==="
@@ -630,6 +693,17 @@ jobs:
             echo "⚠️  Skipping Windows ZIP: Directory artifacts/windows-executable not found"
           fi
 
+          # Copy Linux TAR.GZ (search for any .tar.gz file)
+          if [ -d "artifacts/linux-executable" ]; then
+            copy_artifact \
+              "artifacts/linux-executable" \
+              "*.tar.gz" \
+              "meshcore-sar-${{ steps.metadata.outputs.build_prefix }}-${{ steps.metadata.outputs.timestamp }}-linux.tar.gz" \
+              "Linux TAR.GZ"
+          else
+            echo "⚠️  Skipping Linux TAR.GZ: Directory artifacts/linux-executable not found"
+          fi
+
           # Copy commits data
           if [ -f commits.json ]; then
             echo "✅ Copying commits.json"
@@ -1024,6 +1098,26 @@ jobs:
           HTMLEOF
           fi
 
+          # Add Linux TAR.GZ if exists
+          LINUX_PATH=$(get_latest_file "*-linux.tar.gz")
+          if [ -n "$LINUX_PATH" ]; then
+            LINUX_FILE=$(basename "$LINUX_PATH")
+            LINUX_SIZE=$(get_size "$LINUX_PATH")
+            cat >> "$BUILD_DIR/index.html" <<HTMLEOF
+                <div class="download-card">
+                  <div class="download-info">
+                    <div class="download-platform">
+                      <span class="platform-icon">🐧</span>
+                      <span class="platform-name">Linux (x64)</span>
+                    </div>
+                    <div class="file-name">$LINUX_FILE</div>
+                    <div class="file-size">Size: $LINUX_SIZE</div>
+                  </div>
+                  <a href="$BASE_URL/$LINUX_FILE" class="download-btn" download>Download</a>
+                </div>
+          HTMLEOF
+          fi
+
           # Add commit history section
           if [ -f "$BUILD_DIR/commits.json" ]; then
             # Read commits JSON and embed it directly in the HTML
@@ -1235,6 +1329,11 @@ jobs:
               echo "- 🪟 **Windows ZIP**: [$WIN_FILE]($BASE_URL/$WIN_FILE)" >> $GITHUB_STEP_SUMMARY
             fi
 
+            if [ -f "r2-upload/unstable/${{ steps.metadata.outputs.build_prefix }}"/*-linux.tar.gz ]; then
+              LINUX_FILE=$(basename r2-upload/unstable/${{ steps.metadata.outputs.build_prefix }}/*-linux.tar.gz)
+              echo "- 🐧 **Linux TAR.GZ**: [$LINUX_FILE]($BASE_URL/$LINUX_FILE)" >> $GITHUB_STEP_SUMMARY
+            fi
+
             echo "" >> $GITHUB_STEP_SUMMARY
             echo "- 📋 **Manifest**: [manifest.json]($BASE_URL/manifest.json)" >> $GITHUB_STEP_SUMMARY
             echo "- 📝 **Commits**: [commits.json]($BASE_URL/commits.json)" >> $GITHUB_STEP_SUMMARY
@@ -1245,7 +1344,7 @@ jobs:
   # Create Release on Tag
   create-release:
     name: Create Release
-    needs: [build-android, build-ios, build-macos, build-windows]
+    needs: [build-android, build-ios, build-macos, build-windows, build-linux]
     runs-on: ubuntu-latest
     if: startsWith(github.ref, 'refs/tags/v')
 
@@ -1263,6 +1362,7 @@ jobs:
             artifacts/android-appbundle/*.aab
             artifacts/macos-dmg/*.dmg
             artifacts/windows-executable/*.zip
+            artifacts/linux-executable/*.tar.gz
           draft: true
           generate_release_notes: true
         env: