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/styles/layout.css

@@ -1,14 +1,20 @@
 .site-header {
+  position: sticky;
+  top: 0;
+  z-index: 10;
   display: flex;
   align-items: center;
   justify-content: space-between;
   padding: 1rem 1.5rem;
   border-bottom: 1px solid var(--color-border);
+  background: color-mix(in srgb, var(--color-background) 82%, transparent);
+  backdrop-filter: blur(10px);
 }
 
 .brand {
+  font-family: var(--font-display);
   font-weight: 700;
-  font-size: 1.25rem;
+  font-size: 1.35rem;
   text-decoration: none;
   color: var(--color-foreground);
 }
@@ -37,13 +43,19 @@
 .theme-toggle {
   background: none;
   border: 1px solid var(--color-border);
-  border-radius: 8px;
+  border-radius: 10px;
   width: 44px;
   height: 44px;
   display: inline-flex;
   align-items: center;
   justify-content: center;
   color: var(--color-foreground);
+  transition: border-color 150ms ease, color 150ms ease;
+}
+
+.theme-toggle:hover {
+  border-color: var(--color-primary);
+  color: var(--color-primary);
 }
 
 .site-footer {