Add master display UI with audio output management and styling

- Implement main.js for the master display functionality, including WebSocket connection, audio output management, and state handling.
- Create style.css for the master display's visual design, ensuring a cohesive look and feel with a dark theme and responsive layout.
- Integrate device management with a fallback for non-Electron environments, allowing users to select audio outputs.
- Add features for managing favorites, including toggling favorites and filtering by genre.
- Enhance user experience with a responsive favorites grid and drag-to-scroll functionality.

server/routes/v1.js

@@ -50,7 +50,9 @@ function publicStation(s) {
         country: s.country,
         genres: s.genres,
         description: s.description,
-        image_url: s.image_url,
+        // Prefer the locally-cached file when available so public API consumers
+        // get a stable, fast URL on this host instead of upstream link rot.
+        image_url: s.image_display_url || s.image_url,
         category: s.category,
         enabled: s.enabled,
         up: s.up ?? 0,