feat: add SPA app shell, router, system store, and route placeholders

Nav lists every FC-2 and FC-3 surface with a placeholder view that names
the sub-project that will fill it. System store polls /api/health on mount
and renders a status pip in the nav footer.

Favicon uses Curator's aged amber accent (#A87338).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/system.js

@@ -0,0 +1,18 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+
+export const useSystemStore = defineStore('system', () => {
+  const healthy = ref(null)  // null=unknown, true=ok, false=down
+
+  async function refreshHealth() {
+    try {
+      const r = await fetch('/api/health')
+      const body = await r.json()
+      healthy.value = r.ok && body.status === 'ok'
+    } catch {
+      healthy.value = false
+    }
+  }
+
+  return { healthy, refreshHealth }
+})