Fix Windows build: use powershell instead of pwsh

The runner doesn't have PowerShell Core (pwsh) installed.
Fall back to Windows PowerShell (powershell) which is always available.

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

.gitea/workflows/build-app.yml

@@ -76,7 +76,7 @@ jobs:
     runs-on: windows-latest
     defaults:
       run:
-        shell: pwsh
+        shell: powershell
     steps:
       - name: Checkout
         uses: actions/checkout@v4