stream: make the web player installable as a PWA

Wire up a web manifest, service worker and icon routes so the player can
be installed on mobile. The static manifest stays generic; the page
regenerates it at runtime from STATION_NAME so an instance keeps its name.
The service worker only caches the app shell, never the live stream or the
playback APIs. Icons (192/512, maskable and apple-touch) are rasterized
from the favicon.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

stream/manifest.webmanifest

@@ -0,0 +1,18 @@
+{
+  "name": "Radieo",
+  "short_name": "Radieo",
+  "description": "Radio synthwave en direct.",
+  "lang": "fr",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "background_color": "#0d0b14",
+  "theme_color": "#0d0b14",
+  "icons": [
+    { "src": "/favicon.svg", "sizes": "any", "type": "image/svg+xml" },
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}