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/App.tsx

@@ -18,6 +18,7 @@ import {
   type ThemePrefs,
 } from "./lib/theme";
 import { hasFilterParams, isPage, paramsToFilters, readPage, stripUrlParams, type Page } from "./lib/urlState";
+import { pageTitleKey } from "./lib/pageMeta";
 import {
   loadLayout,
   normalizeLayout,
@@ -284,6 +285,18 @@ export default function App() {
   // Expose the current setPage to decoupled callers (download toast "View", etc.).
   useEffect(() => setNavigator(setPage));
 
+  // Reflect the current module in the browser tab title so open tabs are distinguishable and the
+  // history reads sensibly. Feed = brand only; an open channel page shows the channel name.
+  useEffect(() => {
+    const brand = "Siftlode";
+    const label = channelView
+      ? channelView.name || t("header.channelManager")
+      : page === "feed"
+        ? null
+        : t(pageTitleKey(page));
+    document.title = label ? `${label} · ${brand}` : brand;
+  }, [page, channelView, i18n.language, t]);
+
   function setSidebarLayout(next: SidebarLayout) {
     setSidebarLayoutState(next);
     saveLayoutLocal(next);