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/composables/useBackgroundRefresh.ts

@@ -0,0 +1,29 @@
+import { onMounted, onUnmounted } from 'vue'
+
+/**
+ * Runs `refreshFn` on a recurring interval while the page is visible.
+ * Safe to use in any component — timer is cleared on unmount.
+ *
+ * @param refreshFn   Called each tick. Should be silent (no loading state changes).
+ * @param intervalMs  Polling interval in milliseconds.
+ * @param canRun      Optional guard — refresh is skipped when this returns false.
+ */
+export function useBackgroundRefresh(
+  refreshFn: () => void,
+  intervalMs: number,
+  canRun?: () => boolean,
+): void {
+  let timer: ReturnType<typeof setInterval> | null = null
+
+  onMounted(() => {
+    timer = setInterval(() => {
+      if (document.hidden) return
+      if (canRun && !canRun()) return
+      refreshFn()
+    }, intervalMs)
+  })
+
+  onUnmounted(() => {
+    if (timer !== null) clearInterval(timer)
+  })
+}