ci: gate ESLint + Prettier for frontend and e2e

The new linters/formatters only help if they can't be bypassed. checks.yml now runs
`npm run lint` and `npm run format:check` for both frontend and e2e, alongside the existing
svelte-check / tsc / unit-test gates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/checks.yml

@@ -94,8 +94,16 @@ jobs:
         working-directory: ./frontend
         run: npx svelte-check --threshold error
 
+      - name: ESLint
+        working-directory: ./frontend
+        run: npm run lint
+
+      - name: Prettier
+        working-directory: ./frontend
+        run: npm run format:check
+
   e2e-typecheck:
-    name: E2E — typecheck
+    name: E2E — typecheck + lint
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
@@ -111,3 +119,11 @@ jobs:
       - name: tsc --noEmit
         working-directory: ./e2e
         run: npx tsc --noEmit
+
+      - name: ESLint
+        working-directory: ./e2e
+        run: npm run lint
+
+      - name: Prettier
+        working-directory: ./e2e
+        run: npm run format:check