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/views/ProjectListView.vue

@@ -3,6 +3,7 @@ import { ref, computed, onMounted } from "vue";
 import { useRouter } from "vue-router";
 import { apiGet, apiPost } from "@/api/client";
 import { useToastStore } from "@/stores/toast";
+import { milestoneColor } from "@/utils/palette";
 
 interface MilestoneSummary {
   id: number;
@@ -73,17 +74,6 @@ async function loadProjects() {
   }
 }
 
-function milestoneColor(index: number): string {
-  const palette = [
-    "var(--color-primary)",
-    "var(--color-success)",
-    "#c98a00",
-    "#8b5cf6",
-    "#ef4444",
-    "#06b6d4",
-  ];
-  return palette[index % palette.length];
-}
 
 onMounted(loadProjects);