feat(ci): add Linux AppImage build and upload to desktop workflow

This commit adds support for building and uploading Linux AppImages as part of the desktop platform CI workflow. The build is restricted to the 'modern' matrix label and includes a retry mechanism for stability.

- **feat(ci)**: Added `Build Linux AppImage` step using `:composeApp:packageAppImage` Gradle task.
- **feat(ci)**: Added a `retry` shell function to the AppImage build step to handle transient failures.
- **feat(ci)**: Added `Upload Linux AppImage` step to archive the generated AppImage binaries.
- **chore(ci)**: Increased artifact compression level for Linux installers.

Author: rainxchzed

.github/workflows/build-desktop-platforms.yml

@@ -206,6 +206,34 @@ jobs:
           retry ./gradlew :composeApp:packageDeb :composeApp:packageRpm
         shell: bash
 
+      - name: Build Linux AppImage
+        if: matrix.label == 'modern'
+        run: |
+          set -euo pipefail
+
+          retry() {
+            local n=1
+            local max=3
+            local delay=5
+            while true; do
+              echo "Attempt #$n: $*"
+              if "$@"; then
+                break
+              fi
+              if [ $n -ge $max ]; then
+                echo "Command failed after $n attempts."
+                return 1
+              fi
+              n=$((n+1))
+              echo "Command failed — retrying in $delay seconds..."
+              sleep $delay
+              delay=$((delay*2))
+            done
+          }
+
+          retry ./gradlew :composeApp:packageAppImage
+        shell: bash
+
       - name: Upload Linux installers
         uses: actions/upload-artifact@v4
         with:
@@ -214,4 +242,14 @@ jobs:
             composeApp/build/compose/binaries/main/deb/*.deb
             composeApp/build/compose/binaries/main/rpm/*.rpm
           retention-days: 30
+          compression-level: 6
+
+      - name: Upload Linux AppImage
+        if: matrix.label == 'modern'
+        uses: actions/upload-artifact@v4
+        with:
+          name: linux-appimage
+          path: |
+            composeApp/build/compose/binaries/main/app-image/
+          retention-days: 30
           compression-level: 6