ci: use powershell instead of pwsh on runner

Run 7 failed with 'Cannot find: pwsh in PATH'. The runner has Windows
PowerShell 5.1, not PowerShell 7. Switch explicit-shell steps to
'powershell'; all commands are 5.1-compatible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -35,7 +35,7 @@ jobs:
       # locate it and add it to PATH for subsequent steps. Set a system env var
       # FLUTTER_ROOT to your Flutter SDK dir, or add Flutter's bin to PATH.
       - name: Locate Flutter
-        shell: pwsh
+        shell: powershell
         run: |
           if (Get-Command flutter -ErrorAction SilentlyContinue) {
             Write-Host "flutter already on PATH"; exit 0
@@ -94,11 +94,11 @@ jobs:
         run: flutter build windows --release
 
       - name: Show build output
-        shell: pwsh
+        shell: powershell
         run: Get-ChildItem build\windows\x64\runner\Release
 
       - name: Package artifact
-        shell: pwsh
+        shell: powershell
         run: Compress-Archive -Path "build/windows/x64/runner/Release/*" -DestinationPath "badnote-windows-x64.zip" -Force
 
       - name: Upload artifact (best-effort)