chore: pin Yarn via corepack in release and deploy workflows

The release and deploy-website workflows used threeal/setup-yarn-action with
version: berry, which downloads the latest Yarn (4.17.0) and ignores the
packageManager: yarn@4.14.1 pin. The newer Yarn migrates the lockfile metadata
from 9 to 10, so the install fails with YN0028 (lockfile would be modified),
breaking the release of the previously merged change.

Use corepack with the pinned version, matching the build/test setup action.
deploy-website now installs deps explicitly since the removed action handled it.

Author: merencia

.github/workflows/deploy-website.yml

@@ -30,11 +30,11 @@ jobs:
       - name: Setup Pages
         uses: actions/configure-pages@v4
 
-      - name: Setup Latest Yarn
-        uses: threeal/setup-yarn-action@v2.0.0
-        with:
-          version: berry
-          cache: false
+      - name: Enable Corepack
+        run: corepack enable
+
+      - name: Install dependencies
+        run: yarn install --immutable
 
       - name: Build project
         env:

.github/workflows/release.yml

@@ -36,14 +36,11 @@ jobs:
           node-version: "22"
           registry-url: "https://registry.npmjs.org"
 
-      - name: Setup Latest Yarn
-        uses: threeal/setup-yarn-action@v2.0.0
-        with:
-          version: berry
-          cache: false
+      - name: Enable Corepack
+        run: corepack enable
 
       - name: Install dependencies
-        run: yarn install
+        run: yarn install --immutable
 
       - name: Build
         run: yarn build