CI Windows: use MinGW 13.1 and stage runtime DLLs

- tools_mingw1120 → tools_mingw1310. MinGW 11.2 isn't reliably available
  via aqt anymore; 13.1 is what install-qt-action ships for Qt 6.5.3
  on Windows (confirmed against akashi's working CI config).

- Copy libgcc_s_seh-1.dll / libstdc++-6.dll / libwinpthread-1.dll from
  ${IQTA_TOOLS}/mingw1310_64/bin into ./bin/ after the build. configure.sh
  runs windeployqt with --no-compiler-runtime (matches the pre-migration
  flags), so these need to be staged separately for the artifact to run
  on a machine without MinGW installed.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: OmniTroid

.github/workflows/build.yml

@@ -36,15 +36,22 @@ jobs:
           version: 6.5.3
           target: desktop
           arch: win64_mingw
+          tools: 'tools_mingw1310'
           cache: true
           cache-key-prefix: install-qt-action
           modules: 'qtimageformats qtwebsockets qtmultimedia'
-          tools: 'tools_mingw1120'
 
       - name: Configure and build
         shell: bash
         run: ./configure.sh QT_PATH="$QT_ROOT_DIR" BUILD_TYPE=Release
 
+      - name: Stage MinGW runtime DLLs
+        shell: pwsh
+        run: |
+          Copy-Item "$Env:IQTA_TOOLS\mingw1310_64\bin\libgcc_s_seh-1.dll" bin\
+          Copy-Item "$Env:IQTA_TOOLS\mingw1310_64\bin\libstdc++-6.dll" bin\
+          Copy-Item "$Env:IQTA_TOOLS\mingw1310_64\bin\libwinpthread-1.dll" bin\
+
       - name: Upload Artifact
         uses: actions/upload-artifact@master
         with: