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/utils/palette.ts

@@ -0,0 +1,13 @@
+/** Cyclic color palette for milestone progress bars. */
+export const MILESTONE_PALETTE = [
+  'var(--color-primary)',
+  'var(--color-success, #22c55e)',
+  '#c98a00',
+  '#8b5cf6',
+  'var(--color-danger, #ef4444)',
+  '#06b6d4',
+]
+
+export function milestoneColor(index: number): string {
+  return MILESTONE_PALETTE[index % MILESTONE_PALETTE.length]
+}