feat(ui): new equalizer logo, favicon link, fix favicon path

- icon.svg / icon-192/512.png: blue→purple equalizer gradient design
- nginx.conf: add /app/ alias so reverse-proxy prefix resolves assets
- vite.config.js: base /app/ + %BASE_URL% in favicon links so path
  resolves correctly whether app is served at / or /app/
- ic_launcher.xml: same equalizer design for Android app icon

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx.conf

@@ -35,6 +35,12 @@ server {
         add_header Access-Control-Allow-Origin "*";
     }
 
+    # /app/ alias so the reverse proxy prefix resolves static assets correctly
+    location /app/ {
+        alias /usr/share/nginx/html/;
+        try_files $uri /index.html;
+    }
+
     # SPA fallback
     location / {
         try_files $uri $uri/ /index.html;