Stop tracking the built web UI; build it in the Docker image instead

src/handler/api/static/ was a committed build artifact: Next's content-hashed
chunk names churn on every build, so any two branches touching frontend/ were
guaranteed merge conflicts there, PR diffs drowned in generated churn, and a
forgotten `npm run export` could silently ship a UI older than its source.

- gitignore the export (plus frontend/out and .next were already covered) and
  remove the 52 tracked files.
- Dockerfile grows a `ui` stage (npm ci + npm run build) whose output is copied
  into the packaged tree before pip install, so the image published by docker.yml
  always carries a UI built from exactly that commit's source — the frontend
  build is now effectively part of CI with no new workflow.
- .dockerignore excludes frontend artifacts and any stale local export: COPY
  into src/handler/api/static merges, so a checkout copy must never leak in.
- pyproject: hatchling skips VCS-ignored files, so `artifacts` re-includes the
  export when present; absent it, the wheel builds fine and the API just runs
  headless (it only mounts static/ when the directory exists).
- README documents the two build paths (Docker stage vs `npm run export` for
  source installs) and the headless fallback.

Verified: wheel with the export present ships all 52 files (memory page
included); wheel without it builds clean and create_app() skips the UI mount.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WYqkoYPX8NAo1V2KyXr1pk

.dockerignore

@@ -1,5 +1,6 @@
 # Keep the build context to what the image actually needs: pyproject.toml,
-# README.md (referenced by pyproject), src/, alembic.ini, docker-entrypoint.sh.
+# README.md (referenced by pyproject), src/, frontend/ (source only — the ui stage
+# builds it), alembic.ini, docker-entrypoint.sh.
 .git
 .github
 .venv
@@ -19,3 +20,10 @@ Dockerfile.control
 docker-compose.yml
 .dockerignore
 .mise.toml
+# Frontend artifacts: the ui stage runs npm ci / npm run build itself, and a stale
+# local export must never leak in — COPY into src/handler/api/static merges, so any
+# checkout copy would ship alongside the fresh build.
+frontend/node_modules
+frontend/out
+frontend/.next
+src/handler/api/static