feat(frontend): redesign homepage and file config form with aurora palette

Rebuild the visual identity from the actual Ombrora mark (violet/cyan aurora
blobs) instead of the generic blue/amber scheme, add Bricolage Grotesque and
IBM Plex Mono for display and code-like text, and give the hero an animated
aurora backdrop, gradient headline, and a format-pair marquee driven by the
real registered converters.

Rework the file config form: target format and icon size are both chip
groups now instead of a native select, and quality/compression sliders get a
gradient-filled track, custom thumb, and a mono value pill to match. File
type icons are color-coded by family (images/documents/fonts/ebooks) and
reused across FileConfigCard, FileCard, and FormatsGrid.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/FormatMarquee.jsx

@@ -0,0 +1,31 @@
+import { ArrowRight } from '@phosphor-icons/react';
+
+const PAIRS = [
+  ['heic', 'jpg'],
+  ['png', 'webp'],
+  ['docx', 'pdf'],
+  ['ttf', 'woff'],
+  ['epub', 'mobi'],
+];
+
+function Pill({ from, to }) {
+  return (
+    <span className="format-pill">
+      <span>{from.toUpperCase()}</span>
+      <ArrowRight size={12} weight="bold" />
+      <span>{to.toUpperCase()}</span>
+    </span>
+  );
+}
+
+export function FormatMarquee() {
+  return (
+    <div className="format-marquee" aria-hidden="true">
+      <div className="format-marquee-track">
+        {[...PAIRS, ...PAIRS].map(([from, to], index) => (
+          <Pill key={`${from}-${to}-${index}`} from={from} to={to} />
+        ))}
+      </div>
+    </div>
+  );
+}