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

.gitignore

@@ -223,3 +223,9 @@ __marimo__/
 
 # Handler runtime artifacts
 /handler.db
+
+# Built web UI — a generated artifact, never committed. Local/manual installs produce it
+# with `npm run export` (see README "Frontend"); the Docker image builds it in its own
+# node stage. Tracking it caused guaranteed merge conflicts: Next's content-hashed chunk
+# names churn on every build, so any two branches touching frontend/ collided here.
+/src/handler/api/static/