Initial build: fullhouse, live music bingo from a real Spotify playlist

Based on bingo-bango by f8al (MIT) — same core card-generation idea
("both facets": a square is either an exact song title or an artist,
and an artist square lights up on any of their tracks), reworked with
a real host/guest split, live multiplayer sessions, and cover-art
squares. f8al credited in the footer, README, and PROPOSAL.md.

Three ways to play: host a live session (server-side Spotify
connection, join code + QR, caller screen, auto-call from real
playback), join someone else's session (no login needed), or solo
(guest PKCE login or a pasted public playlist URL, one-off card).

Genuinely verified against a real Spotify account throughout, not
just built and assumed working — including two real bugs found and
fixed along the way (Spotify's playlist-tracks endpoint quietly
renamed to /items with a reshaped response; reading playlist tracks
needs real user auth even for public playlists, so the "no guest
login" public-playlist path routes through the host's connection
instead of a dead-end app-only token). Both Docker images built and
run together on a real network with the nginx proxy verified working
end-to-end, and auto-call confirmed detecting an actual track playing
live through Spotify Connect within one poll tick.

Dockerfile

@@ -0,0 +1,24 @@
+# ── Stage 1: build the Vite client ────────────────────────────────────────
+FROM node:22-alpine AS build
+
+WORKDIR /app/client
+COPY client/package.json client/package-lock.json* ./
+RUN npm install
+COPY client/ .
+
+# Vite bakes VITE_* vars in at build time, not runtime — has to be a
+# real build ARG, not just a container env var set later.
+ARG VITE_SPOTIFY_CLIENT_ID
+ENV VITE_SPOTIFY_CLIENT_ID=${VITE_SPOTIFY_CLIENT_ID}
+# Deliberately empty: relies on nginx same-origin-proxying /api to the
+# api service (see nginx.conf) instead of an absolute cross-origin URL
+# — the CORS bug that broke local dev earlier in this build.
+ENV VITE_API_BASE=
+
+RUN npx tsc && npx vite build
+
+# ── Stage 2: serve ─────────────────────────────────────────────────────────
+FROM nginx:alpine
+COPY --from=build /app/client/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80