Phase 4: drone + T6/E-series + polish + deploy + README

Add drone synth + DronePlayer, wire T6 (mode over drone) and E1-E3 info pages
(E3 carries a drone). Screen Wake Lock while any click/drone plays. Vitest suite
for scheduler timing (no drift over 5 min) and music-layer correctness (triads,
CAGED, diatonic, fretboard). Deploy artifacts: public/_redirects, multi-stage
Dockerfile + nginx.conf (SPA fallback, immutable asset caching, gzip),
.dockerignore. README covering build, Cloudflare Pages / nginx / Docker deploy,
and the deep-link URL format. prefers-reduced-motion honored throughout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,20 @@
+# --- Build stage ---------------------------------------------------------------
+FROM node:22-alpine AS build
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+
+# Base path baked at build time. Default '/' (dedicated domain).
+# For a subpath deploy: docker build --build-arg VITE_BASE=/practice/ .
+ARG VITE_BASE=/
+ENV VITE_BASE=$VITE_BASE
+RUN npm run build
+
+# --- Serve stage ---------------------------------------------------------------
+FROM nginx:alpine
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx/default.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80