Fix oscilloscope tracks: transcode N-Spheres FLAC to MP3

The 5 N-Spheres tracks shipped as full-length FLAC (30-52MB each, ~189MB
total) and failed to play in Safari/iOS (flaky FLAC support). Transcode to
192kbps MP3 (~41MB total, stereo preserved for the XY Lissajous) which plays
everywhere. Matches the working spirals_shrt.mp3 pattern.

frontend/src/components/OscilloscopeDisplay.astro

@@ -125,11 +125,11 @@
   // ── Signal configuration ──────────────────────────────────
   const SIGNALS = [
     { id: 'spirals',   name: 'Spirals',   src: '/spirals_shrt.mp3', artist: 'Jerobeam Fenderson', license: 'CC BY-NC-SA 4.0', link: 'http://oscilloscopemusic.com/' },
-    { id: 'function',  name: 'Function',  src: '/n-spheres/function.flac',  artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
-    { id: 'intersect', name: 'Intersect', src: '/n-spheres/intersect.flac', artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
-    { id: 'attractor', name: 'Attractor', src: '/n-spheres/attractor.flac', artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
-    { id: 'flux',      name: 'Flux',      src: '/n-spheres/flux.flac',      artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
-    { id: 'core',      name: 'Core',      src: '/n-spheres/core.flac',      artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
+    { id: 'function',  name: 'Function',  src: '/n-spheres/function.mp3',  artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
+    { id: 'intersect', name: 'Intersect', src: '/n-spheres/intersect.mp3', artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
+    { id: 'attractor', name: 'Attractor', src: '/n-spheres/attractor.mp3', artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
+    { id: 'flux',      name: 'Flux',      src: '/n-spheres/flux.mp3',      artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
+    { id: 'core',      name: 'Core',      src: '/n-spheres/core.mp3',      artist: 'Fenderson & Hansi3D', album: 'N-Spheres' },
   ] as const;
 
   // ── Skin configuration ────────────────────────────────────