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/NavSidebar.tsx

@@ -256,8 +256,9 @@ export default function NavSidebar({
         {!collapsed && (
           <button
             onClick={() => setPage("feed")}
-            className="text-lg font-bold tracking-tight select-none"
+            className="text-lg font-bold tracking-tight select-none cursor-pointer rounded-md -mx-1 px-1 hover:bg-card hover:opacity-90 transition"
             title={t("header.feed")}
+            aria-label={t("header.feed")}
           >
             Sift<span className="text-accent">lode</span>
           </button>