Add Sycamore/WASM frontend and restructure into server/ + web/

Repo is now a monorepo:

- server/: the kaya backend, unchanged in behaviour, plus:
  - GET /api/me for SPA session detection
  - last_move recorded on every play and broadcast in the game state, so
    clients can show who played which card the moment they play it
  - legal_moves per hand card for the player on turn (rules stay
    server-side)
  - static catch-all route serving the compiled SPA with index.html
    fallback; Tortoise context now bound only for /api/* requests
  - configurable OIDC post-login/logout redirects for dev against trunk
- web/: Sycamore 0.9 + WASM frontend (trunk): login via the OIDC flow,
  lobby (create match / join by code), live game page over websocket with
  card images (CC0 woodcut napoletane deck), capture picker, move banner,
  game-over overlay, match history and leaderboard pages
- server/Dockerfile gains a rust+trunk stage building web/dist; the single
  app image serves the SPA; compose builds from the repo root with
  overridable ports/OIDC env

Verified end-to-end against the compose stack: four OIDC logins, game
creation, three joins by code, websocket play with broadcasts to all
players and out-of-turn rejection. 51 backend tests, mypy and cargo tests
all green.

.dockerignore

@@ -1,21 +1,19 @@
-# Keep the build context lean: exclude the local venv, caches, tests,
-# and anything not needed to build the scopa wheel.
+# Build context is the repository root (see docker-compose.yml). Keep it
+# lean: venvs, caches, Rust build output and tests are not needed to build
+# the image.
 .git/
 .gitignore
 .env
 .env.example
-.venv/
-__pycache__/
+**/.venv/
+**/__pycache__/
 *.pyc
 *.pyo
 *.egg-info/
 .mypy_cache/
 .pytest_cache/
-dist/
-build/
-conf/
-tests/
-Dockerfile
-.dockerignore
-opencode.json
+web/target/
+web/dist/
+server/tests/
 docker-compose.yml
+opencode.json