refactor: DRY pass on frontend — shared palette, composable, and tab loader

- Extract milestoneColor to utils/palette.ts; remove duplicate in HomeView + ProjectListView
- Create useBackgroundRefresh composable; wire into HomeView + BriefingView (removes manual setInterval/clearInterval boilerplate)
- Extract _loadTabContent() in SettingsView so watch and onMounted share one tab→loader mapping
- Move raw fetch() api-key calls to typed helpers in api/client.ts (listApiKeys, createApiKey, revokeApiKey)
- Drop unused onUnmounted import from BriefingView

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -580,3 +580,21 @@ export async function updateEvent(id: number, payload: EventUpdatePayload): Prom
 export async function deleteEvent(id: number): Promise<void> {
   return apiDelete(`/api/events/${id}`);
 }
+
+// ─── API Keys ─────────────────────────────────────────────────────────────────
+
+export interface ApiKeyEntry {
+  id: number
+  name: string
+  scope: string
+  key_prefix: string
+  last_used_at: string | null
+}
+
+export const listApiKeys = () =>
+  apiGet<{ api_keys: ApiKeyEntry[] }>('/api/api-keys').then(r => r.api_keys)
+
+export const createApiKey = (name: string, scope: 'read' | 'write') =>
+  apiPost<{ key: string; api_key: ApiKeyEntry }>('/api/api-keys', { name, scope })
+
+export const revokeApiKey = (id: number) => apiDelete(`/api/api-keys/${id}`)