build(release): split core builds from release-only artifacts

- add a dedicated build-release-artifacts target for Android bundle packaging
- switch CI and release workflows to Corepack-managed pnpm with cache support
- pin the frontend pnpm version and make dependency installs deterministic
- inject version metadata into launcher binaries in GoReleaser
- update build documentation to reflect the new workflow

.github/workflows/build.yml

@@ -16,5 +16,15 @@ jobs:
         with:
           go-version-file: go.mod
 
+      - name: Setup Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: 22
+          cache: pnpm
+          cache-dependency-path: web/frontend/pnpm-lock.yaml
+
+      - name: Setup pnpm
+        run: corepack enable && corepack install
+
       - name: Build
         run: make build-all