ci: consolidate web and android workflows into one pipeline

Both subprojects ship from the same commit, so ordinary CI is now a single
ci.yml; only the tag-driven release stands apart. The web app is built twice
per run — once per base path — and every consumer downloads the artifact
instead of rebuilding, replacing three redundant base-"" builds on main.

Nothing deploys unless the test job is green, and android-release runs the
suite before signing (ci.yml does not fire on tags, so it was the only gap).

Shared toolchain setup moves into composite actions, which puts the web build
and the APK on the same Node version for the first time. The Firebase PR path
was still on npm ci against a stale web/package-lock.json that could resolve a
different tree than pnpm-lock.yaml; drop the lockfile and the npm path with it.

Also: least-privilege permissions widened per job, persist-credentials off on
every checkout, concurrency groups that cancel superseded PRs but never a live
deploy, npm caching for android, and the Firebase action pinned by commit SHA
to match how the release actions were already pinned.

.github/workflows/android-release.yml

@@ -1,3 +1,7 @@
+# Tag-driven release. Builds standalone rather than consuming ci.yml
+# artifacts: a tag run has no upstream run to pull from, and a release
+# should be reproducible from the tagged tree alone.
+
 name: android / release
 
 on:
@@ -7,40 +11,36 @@ on:
 permissions:
   contents: write
 
-jobs:
-  build:
-    runs-on: ubuntu-latest
+concurrency:
+  group: android-release-${{ github.ref }}
+  cancel-in-progress: false
 
-    defaults:
-      run:
-        working-directory: android
+jobs:
+  release:
+    runs-on: ubuntu-latest
 
     steps:
       - uses: actions/checkout@v7
         with:
           persist-credentials: false
 
-      - name: Set up Node 22
-        uses: actions/setup-node@v7
-        with:
-          node-version: '22'
+      - uses: ./.github/actions/setup-web
 
-      - name: Set up JDK 21
-        uses: actions/setup-java@v5
-        with:
-          distribution: temurin
-          java-version: '21'
+      # ci.yml does not run on tags, so this is the only gate before signing.
+      - name: Test web
+        run: pnpm test
+        working-directory: web
 
-      - name: Set up Gradle
-        uses: gradle/actions/setup-gradle@v6
-        with:
-          cache-provider: basic
+      # Base "" — the APK loads the bundle from the domain root.
+      - name: Build web
+        run: pnpm build
+        working-directory: web
 
-      - name: Install wrapper deps
-        run: npm ci
+      - uses: ./.github/actions/setup-android
 
-      - name: Build web + sync into Android
-        run: npm run build
+      - name: Sync web bundle into the native project
+        run: npm run sync
+        working-directory: android
 
       - name: Decode keystore
         env: