feat(ui): favicon, dynamic tab title, clickable logo, styled module headers

Session-close cosmetics:
1. Favicon — an 'S' monogram (indigo→violet rounded square, path-drawn so it's font-independent)
   at public/favicon.svg, linked in index.html (tab had none before).
2. Dynamic document.title — reflects the current module ('Downloads · Siftlode'); Feed = brand
   only; an open channel page shows the channel name. Shared pageTitleKey() in lib/pageMeta.ts
   keeps the tab title and the top-bar header in lockstep.
3. The 'Siftlode' logo already navigated to Feed but read as static text — added a hover
   background + cursor affordance so it's clearly clickable.
4. Module header — new shared PageTitle component (used by every non-feed module via Header) with
   a tracked small-caps 'eyebrow' + leading accent dot (user-picked style B), styled in one place.
Verified in a real browser: favicon served (image/svg+xml), title updates per module, logo→feed,
header restyled; no console errors.

frontend/src/components/PageTitle.tsx

@@ -0,0 +1,11 @@
+// The centered module title in the top bar. Every non-feed module renders through this one
+// component, so its styling lives in a single place. Design element (user-picked): a tracked
+// small-caps "eyebrow" with a leading accent dot.
+export default function PageTitle({ label }: { label: string }) {
+  return (
+    <span className="inline-flex items-center gap-2 text-xs font-semibold uppercase tracking-[0.16em] text-fg">
+      <span className="w-1.5 h-1.5 rounded-full bg-accent shrink-0" />
+      {label}
+    </span>
+  );
+}