Provision pnpm via Corepack in CI to honor packageManager pin

CI installed pnpm with 'npm install -g pnpm', which pulls the latest
release and ignores the pnpm@10.33.0 pin in package.json's
packageManager field. This let CI drift to a different pnpm major than
the Docker image and local dev, the exact drift Corepack prevents.

Switch both the GitHub and Gitea build workflows to 'corepack enable',
matching the Dockerfile assets stage and the documented model where
local, CI, and Docker all follow the packageManager field.

https://claude.ai/code/session_01VWXYQxUPWdhoV4otwr6Cyk

.gitea/workflows/build.yml

@@ -25,7 +25,7 @@ jobs:
           node-version: "22"
 
       - name: Install pnpm and JS dependencies
-        run: npm install -g pnpm && pnpm install --frozen-lockfile --ignore-scripts
+        run: corepack enable && pnpm install --frozen-lockfile --ignore-scripts
 
       - name: Build TypeScript
         run: make ts

.github/workflows/build-docker.yml

@@ -25,7 +25,7 @@ jobs:
           node-version: "22"
 
       - name: Install pnpm and JS dependencies
-        run: npm install -g pnpm && pnpm install --frozen-lockfile --ignore-scripts
+        run: corepack enable && pnpm install --frozen-lockfile --ignore-scripts
 
       - name: Build TypeScript
         run: make ts