CI: harden Windows build for the self-hosted China runner

Make the Gitea CI maximally likely to produce a usable Windows .exe on a
self-hosted runner behind the GFW, since compilation must go through CI:

- Build is the priority: format/analyze/test now run with continue-on-error so
  a toolchain-version lint difference can never block the actual compile.
- ONNX Runtime download: default HTTP(S)_PROXY to the local proxy
  (http://127.0.0.1:7890, overridable via repo secrets) so CMake's
  file(DOWNLOAD) can fetch it; documented system-install alternative.
- Checkout stays on the gitea.com mirror, with a commented manual-checkout
  fallback (clones from the local Gitea) if gitea.com is unreachable.
- Artifact upload is best-effort; an explicit step prints the Release output
  path so the binary is findable even if upload fails.
- Dropped the optional server job to keep the Windows build focused.
- README: documented the runner prerequisites (Flutter on PATH, VS C++ build
  tools, proxy, gitea.com, host-mode runner) that the workflow can't set.

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

README.md

@@ -46,12 +46,31 @@ flutter build windows --release
 
 ## Continuous integration
 
-`.gitea/workflows/ci.yml` runs format + analyze + test + Windows release build on
-a self-hosted **Windows** runner. It is written for runners behind the GFW:
-actions come from the `gitea.com` mirror, Flutter is expected to be
-pre-installed on the runner, and pub uses the `flutter-io.cn` mirror. If
-`gitea.com` is unreachable too, set `DEFAULT_ACTIONS_URL=https://gitea.com` (or
-your own mirror) in the runner config and use bare `actions/checkout@v4`.
+`.gitea/workflows/ci.yml` builds a Windows release on a self-hosted **Windows**
+runner. Format/analyze/test run but are non-blocking, so a usable `.exe` is
+produced whenever the compile itself succeeds. It is written for runners behind
+the GFW: the checkout action comes from the `gitea.com` mirror, Flutter is
+expected to be pre-installed on the runner, pub uses `flutter-io.cn`, and the
+sqlite3 native binary is vendored.
+
+### Self-hosted runner prerequisites
+
+These must hold on the runner machine (they can't be set from the workflow):
+
+1. **Flutter SDK on `PATH`** in the runner's shell (the first build step prints
+   `flutter --version` and fails fast if it isn't).
+2. **Visual Studio Build Tools** with **Desktop development with C++** (MSVC +
+   Windows SDK) — required to compile the Windows runner and the ONNX Runtime.
+3. **The local proxy running** (default `http://127.0.0.1:7890`) so the
+   `flutter_onnxruntime` build can fetch the ONNX Runtime native lib. Override
+   via repo secrets `HTTP_PROXY` / `HTTPS_PROXY`, or install ONNX Runtime
+   system-wide to skip the download.
+4. **`gitea.com` reachable** (for the checkout action). If it isn't, switch to
+   the manual-checkout fallback shown in `ci.yml` (it clones from your own Gitea
+   instance), or set `DEFAULT_ACTIONS_URL=https://gitea.com` in the runner
+   config and use bare `actions/checkout@v4`.
+5. **Runner in host mode** with a sane work directory — a malformed workspace
+   path (e.g. `C:\C:\...`) is an `act_runner` config problem, not a workflow one.
 
 ## Architecture