Fix Linux AppImage bundling and Windows Rust install

- Linux: add xdg-utils to system deps (provides xdg-open needed by
  Tauri's AppImage bundler)
- Windows: replace dtolnay/rust-toolchain action (uses bash internally)
  with direct rustup install via PowerShell
- Unix: install Rust via rustup.rs shell script instead of GitHub action

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -120,14 +120,29 @@ jobs:
         with:
           node-version: ${{ env.NODE_VERSION }}
 
-      - name: Install Rust stable
+      - name: Install Rust stable (Unix)
-        uses: dtolnay/rust-toolchain@stable
+        if: matrix.platform != 'windows'
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable
+          echo "$HOME/.cargo/bin" >> $GITHUB_PATH
+
+      - name: Install Rust stable (Windows)
+        if: matrix.platform == 'windows'
+        shell: powershell
+        run: |
+          if (Get-Command rustup -ErrorAction SilentlyContinue) {
+            rustup default stable
+          } else {
+            Invoke-WebRequest -Uri https://win.rustup.rs/x86_64 -OutFile rustup-init.exe
+            .\rustup-init.exe -y --default-toolchain stable
+            echo "$env:USERPROFILE\.cargo\bin" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append
+          }
 
       - name: Install system dependencies (Linux)
         if: matrix.platform == 'linux'
         run: |
           sudo apt-get update
-          sudo apt-get install -y libgtk-3-dev libwebkit2gtk-4.1-dev libappindicator3-dev librsvg2-dev patchelf
+          sudo apt-get install -y libgtk-3-dev libwebkit2gtk-4.1-dev libappindicator3-dev librsvg2-dev patchelf xdg-utils
 
       - name: Install system dependencies (macOS)
         if: matrix.platform == 'macos'