Fix Linux build PATH and add ARM64 container support

Linux app build: cargo was not in PATH for subsequent steps after
shell-based install. Fixed by adding $HOME/.cargo/bin to GITHUB_PATH
(persists across steps) and setting it in the job-level env. Also
removed the now-unnecessary per-step PATH override in the macOS job.

Container build: added QEMU setup and platforms: linux/amd64,linux/arm64
to produce a multi-arch manifest. The Dockerfile already uses
arch-aware commands (dpkg --print-architecture, uname -m) so it
builds natively on both architectures. This fixes the "no matching
manifest for linux/arm64/v8" error on Apple Silicon Macs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -21,6 +21,9 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
@@ -36,6 +39,7 @@ jobs:
         with:
           context: ./container
           file: ./container/Dockerfile
+          platforms: linux/amd64,linux/arm64
           push: ${{ gitea.event_name == 'push' }}
           tags: |
             ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest