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/public/favicon.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <defs>
+    <linearGradient id="s" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#7c86ff"/>
+      <stop offset="1" stop-color="#a855f7"/>
+    </linearGradient>
+  </defs>
+  <rect width="64" height="64" rx="16" fill="url(#s)"/>
+  <!-- Bold "S" as a path so it renders identically without depending on an installed font. -->
+  <path fill="#ffffff" d="M41.8 22.4c-2.2-2.5-5.6-4-9.9-4-6.3 0-10.6 3.3-10.6 8.4 0 4.9 3.6 7 9.4 8.2 4.4.9 5.9 1.7 5.9 3.5 0 1.8-1.8 2.9-4.8 2.9-3 0-5.3-1.2-7-3.3l-4.8 4c2.4 3.1 6.4 5 11.6 5 6.8 0 11.3-3.4 11.3-8.9 0-5.2-3.7-7.2-9.7-8.4-4.2-.8-5.6-1.5-5.6-3.2 0-1.6 1.6-2.6 4.2-2.6 2.6 0 4.6 1 6.1 2.8z"/>
+</svg>