overhaul: single-session deployment + redesigned frontend

Backend simplification:
- The server now loads ONE pool JSON from $QUIZ_POOL_PATH at startup and
  upserts a single canonical session. The session id comes from the pool
  JSON's optional "session_id" field, falling back to $QUIZ_SESSION_ID.
- The multi-quiz / multi-session CRUD API is gone:
    DELETED  GET/POST /admin/api/quizzes
    DELETED  POST    /admin/api/quizzes/upload
    DELETED  GET/POST /admin/api/sessions
    DELETED  GET     /admin/login (HTML stub)
    DELETED  GET     /admin/api/sessions/{sid}/csv (replaced by /admin/api/csv)
  Replaced with a single-session control surface:
    GET  /admin/                — serves admin.html unconditionally
    GET  /admin/api/state       — admin-gated; pool meta + state + QR + join URL
    POST /admin/api/reset       — admin-gated; wipe submissions + back to lobby
    POST /admin/logout          — clear admin cookie
    GET  /admin/api/csv         — single-session results
    WS   /ws/instructor/{sid}   — kept; new commands "next" + "reset"
- Instructor "Next" button is now a single state-driving command
  (RoomManager.advance_to_next): from lobby it opens Q0; from question_open
  it closes the current Q and opens the next; from question_closed it
  opens the next; if past the last question it ends the session.
- New RoomManager.reset wipes submissions, participants, and per-question
  state, then broadcasts a clean lobby.
- Student GET / now redirects to /?sid=<canonical> when no sid is given,
  so the QR / share URL is fully deterministic.

Frontend rewrite (functional baseline; visual polish to follow):
- /admin/ is now a single SPA: GET /admin/api/state decides login form
  vs dashboard. No separate /admin/login URL bar.
- Admin dashboard is state-driven with one primary action per state.
  QR code, join URL, and live participant list are always visible on the
  left so the operator can leave the page on a projector.
- Student answer buttons are big and tappable; reveal screen highlights
  correct/wrong choice + shows score, total, and rank.
- Static admin/student SPAs share a CSS palette with light/dark support.

Tests rewritten around the single canonical session id.
The auto-bootstrapped session lets each test fixture skip the old
quiz/session creation dance. 39/39 tests pass.

Cleanup:
- Deleted CODEX_PROMPT.md, IMPLEMENTATION_REPORT.md, NOTES.md, SPEC.md,
  static/observer.html (obsolete codex-build artifacts and the unused
  observer view).
- .gitignore now blocks /pool.json (the runtime file the operator drops
  on the server) and the leftover .codex_done / codex_run.log / etc.
- bootstrap.sh seeds /opt/quiz/pool.json from examples/pool_example.json
  on first deploy so a fresh box reaches a usable state without manual
  intervention; .env now includes QUIZ_POOL_PATH.

tests/test_csv_export.py

@@ -1,8 +1,8 @@
-from conftest import create_session, join_student
+from conftest import admin_login, join_student
 
 
-def test_csv_export_contains_one_row_per_submission(client, sample_pool):
-    sid = create_session(client, sample_pool)
+def test_csv_export_contains_one_row_per_submission(client, sid):
+    admin_login(client)
     join_student(client, sid, "s1", "Student One")
     rooms = client.app.state.rooms
     client.portal.call(rooms.open_question, sid, 0, 2)
@@ -10,7 +10,7 @@ def test_csv_export_contains_one_row_per_submission(client, sample_pool):
     assert ack["type"] == "submit_ack"
     client.portal.call(rooms.close_question, sid)
 
-    response = client.get(f"/admin/api/sessions/{sid}/csv")
+    response = client.get("/admin/api/csv")
     lines = response.text.strip().splitlines()
     assert lines[0] == "sid,student_id,name,question_idx,answer,elapsed_ms,score,status"
     assert len(lines) == 2