CI: locate Flutter on the runner instead of assuming it is on PATH

Root cause of the failing Windows runs (seen in the runner logs): the Actions
shell does not inherit Flutter on PATH, so `flutter --version` failed in ~30s.
Checkout via the gitea.com mirror works fine.

Add a "Locate Flutter" step that checks PATH, the FLUTTER_ROOT repo variable,
and common Windows install locations, then prepends it via GITHUB_PATH so later
steps find flutter. Fails with a clear message if not found.

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

.gitea/workflows/ci.yml

@@ -44,7 +44,36 @@ jobs:
         #     git -c http.extraheader="AUTHORIZATION: basic $h" fetch --depth=1 origin "${{ github.sha }}"
         #     git checkout --force FETCH_HEAD
 
-      - name: Flutter version (must be pre-installed on the runner)
+      # The runner's Actions shell often does not inherit Flutter on PATH, so
+      # locate it and add it to PATH for subsequent steps. Override by setting
+      # the repo variable FLUTTER_ROOT (Settings > Actions > Variables) to your
+      # Flutter SDK directory, or just add Flutter's bin to the system PATH.
+      - name: Locate Flutter
+        shell: pwsh
+        run: |
+          if (Get-Command flutter -ErrorAction SilentlyContinue) {
+            Write-Host "flutter already on PATH"; exit 0
+          }
+          $candidates = @()
+          if ("${{ vars.FLUTTER_ROOT }}") { $candidates += (Join-Path "${{ vars.FLUTTER_ROOT }}" 'bin') }
+          if ($env:FLUTTER_ROOT) { $candidates += (Join-Path $env:FLUTTER_ROOT 'bin') }
+          $candidates += @(
+            'C:\flutter\bin','C:\src\flutter\bin','C:\tools\flutter\bin','C:\dev\flutter\bin',
+            "$env:USERPROFILE\flutter\bin","$env:USERPROFILE\development\flutter\bin",
+            "$env:LOCALAPPDATA\flutter\bin",'C:\fvm\default\bin'
+          )
+          $found = $null
+          foreach ($c in $candidates) {
+            if ($c -and (Test-Path (Join-Path $c 'flutter.bat'))) { $found = $c; break }
+          }
+          if (-not $found) {
+            Write-Error "Flutter not found on PATH or in common locations. Add Flutter's bin to the runner PATH, or set repo variable FLUTTER_ROOT to your Flutter SDK directory."
+            exit 1
+          }
+          Write-Host "Found Flutter at $found"
+          Add-Content -Path $env:GITHUB_PATH -Value $found
+
+      - name: Flutter version
         run: flutter --version
 
       - name: Install dependencies