swarm-ui: a start page at / and the hive directory at /hives

/ is now the swarm's name over the digital-rain backdrop the hive
dashboard's home has, and the swarm name in the header links to it. The
hive directory moves to /hives, where the nav's hives tab points. The
swarm name is read once per page load by a shared hook, the header and
the start page both use it. Closes #5252

frontend/packages/swarm-ui/src/shell/Shell.css

@@ -55,6 +55,12 @@
   -webkit-backdrop-filter: blur(8px) saturate(120%);
   backdrop-filter: blur(8px) saturate(120%);
 }
+/* The swarm name links to the start page; the link adds no look of its own,
+   so the brand's accent and glow (on the span inside) are all that show. */
+.shell-brand-link {
+  color: inherit;
+  text-decoration: none;
+}
 .shell-brand {
   font-weight: 600;
   /* Base colour only — Shell.tsx overrides it inline to the same