add web UI: search, download queue with progress, cancel

- ui/index.html, ui/styles.css, ui/scripts.js — minimal two-tab UI
- Search tab: big search bar, results with size/votes, add-to-queue button
- Queue tab: progress bar, speed, downloaded/total, status badges, cancel
- Switches to queue tab on load if downloads are active
- Backend: GET /api/search, POST /api/queue/add, GET /api/queue, POST /api/queue/cancel
- Speed tracking in streamToFile, abort tracking for immediate cancel
- No rounded corners, dark theme

Dockerfile

@@ -3,5 +3,6 @@ WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci --omit=dev
 COPY src/ ./src/
+COPY ui/ ./ui/
 EXPOSE 3001
 CMD ["node", "src/index.js"]