Header: use the app icon instead of the "TS" text badge (links home)

The header showed a plain "TS" text badge while the real brand icon
(public/icon.svg — the yellow knowledge-graph tile) was only used as the
favicon/PWA icon. Swap the badge for the actual icon, and make the logo +
name a RouterLink to the board (standard "logo goes home").

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/AppShell.vue

@@ -230,12 +230,10 @@ async function signOut() {
             <line x1="3" y1="18" x2="21" y2="18" />
           </svg>
         </button>
-        <div class="flex shrink-0 items-center gap-2">
-          <div class="flex h-8 w-8 items-center justify-center rounded-lg bg-brand text-sm font-black text-neutral-900">
-            TS
-          </div>
+        <RouterLink to="/" class="flex shrink-0 items-center gap-2" title="Home" aria-label="Home">
+          <img src="/icon.svg" alt="" class="h-8 w-8 rounded-lg" width="32" height="32" />
           <span class="hidden font-semibold sm:inline">{{ config.siteName }}</span>
-        </div>
+        </RouterLink>
 
         <div class="flex flex-1 justify-center">
           <input