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.

web/Trunk.toml

@@ -0,0 +1,18 @@
+# Development server configuration for `trunk serve`.
+# The SPA runs on :8000 and proxies API, auth and websocket traffic to the
+# backend on :8080 so the session cookie works without CORS.
+[serve]
+port = 8000
+
+[[proxy]]
+rewrite = "/api"
+backend = "http://127.0.0.1:8080/api"
+
+[[proxy]]
+rewrite = "/auth"
+backend = "http://127.0.0.1:8080/auth"
+
+[[proxy]]
+rewrite = "/ws"
+backend = "http://127.0.0.1:8080/ws"
+ws = true