docs(images): correct misleading AVIF default in RustyImage / TopBanner

The runtime default has been `["webp", "jpeg"]` for a while, but the doc
comments still referenced AVIF. Update them and add the rationale (server-side
AVIF encoding is 5-10× slower than WebP for marginal bandwidth gain).

No runtime change.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/RustyImage.svelte

@@ -32,9 +32,9 @@
     style?: string;
     /** Pixel-Dichten fürs srcset. Default [1, 2] (Retina). */
     densities?: number[];
-    /** Format-Fallback-Kette. Default ['avif','webp','jpeg']. */
+    /** Format-Fallback-Kette. Default ['webp','jpeg']. AVIF bewusst ausgelassen — server-seitiger Encoder ist 5-10× langsamer als WebP, Bandbreitengewinn marginal. */
     formats?: ("avif" | "webp" | "jpeg")[];
-    /** JPEG/AVIF/WebP-Qualität. Default 75. */
+    /** JPEG/WebP-Qualität. Default 75. */
     quality?: number;
     fit?: "cover" | "contain" | "fill";
     /** Cache-Bust-Tag (z.B. post._updated) → `_v`-Param, bricht Browser-HTTP-Cache. */