web: fix OIDC navigation intercepted by the SPA router; tidy table layout

- Login/Logout anchors now carry rel="external": sycamore-router
  intercepts same-origin anchor clicks otherwise, so the OIDC login
  round-trip never left the SPA (found with a real browser test)
- Opponents' hidden hands render as a tidy 5x2 grid of card backs, seats
  have a fixed height and shrink-wrap so the table no longer jumps as
  cards are played
- Dockerfile: cache the Rust target dir so frontend rebuilds are fast
- ignore .playwright-mcp/ tooling output

server/Dockerfile

@@ -34,6 +34,7 @@ COPY web/assets ./assets
 COPY web/src ./src
 
 RUN --mount=type=cache,target=/usr/local/cargo/registry \
+    --mount=type=cache,target=/web/target \
     trunk build --release
 
 # --- Python builder ----------------------------------------------------------