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/lib/pageMeta.ts

@@ -0,0 +1,32 @@
+import type { Page } from "./urlState";
+
+// The i18n key for a page's human title — shared by the top-bar header and the browser tab title
+// so the two never drift. Keep in sync with the nav modules in NavSidebar / App's page switch.
+export function pageTitleKey(page: Page): string {
+  switch (page) {
+    case "feed":
+      return "header.account.feed";
+    case "channels":
+      return "header.channelManager";
+    case "playlists":
+      return "header.account.playlists";
+    case "notifications":
+      return "inbox.navLabel";
+    case "messages":
+      return "messages.navLabel";
+    case "downloads":
+      return "downloads.navLabel";
+    case "stats":
+      return "header.usageStats";
+    case "scheduler":
+      return "header.scheduler";
+    case "config":
+      return "header.configuration";
+    case "users":
+      return "header.users";
+    case "settings":
+      return "settings.title";
+    default:
+      return "header.account.feed";
+  }
+}