Finish the mobile performance pass

- Cache the model catalog once per user and each conversation's setting
  separately; clear both on logout and after model saves
- Compress hashed assets asynchronously with a byte budget and set Vary
- Memoize message parsing and throttle selection reads
- Debounce draft writes to localStorage
- Keep the composer editor on the page and focused, as before
- Add a bundle budget check and run it in CI
- Show a loading block while sessions load and a workspace avatar in the
  mobile conversation header

.github/workflows/ci.yml

@@ -33,3 +33,9 @@ jobs:
 
       - name: Test
         run: bun run test
+
+      - name: Build pages
+        run: NODE_ENV=production bunx bunext build
+
+      - name: Check bundle budget
+        run: bun run check:bundle