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/HomeView.vue

@@ -1,6 +1,8 @@
 <script setup lang="ts">
 import { ref, computed, onMounted, onUnmounted } from "vue";
 import { apiGet, listEvents } from "@/api/client";
+import { useBackgroundRefresh } from "@/composables/useBackgroundRefresh";
+import { milestoneColor } from "@/utils/palette";
 import type { Note } from "@/types/note";
 import type { Task, TaskListResponse, TaskStatus } from "@/types/task";
 import type { ToolCallRecord, Message } from "@/types/chat";
@@ -69,24 +71,8 @@ const upcomingEvents = ref<EventEntry[]>([]);
 const eventSlideOverOpen = ref(false);
 const editingEvent = ref<EventEntry | null>(null);
 
-// ─── Milestone color palette ──────────────────────────────────────────────────
-
-function milestoneColor(index: number): string {
-  const palette = [
-    "var(--color-primary)",
-    "var(--color-success, #22c55e)",
-    "#c98a00",
-    "var(--color-danger, #e74c3c)",
-    "#8b5cf6",
-  ];
-  return palette[index % palette.length];
-}
-
 // ─── Background refresh (no-flicker) ─────────────────────────────────────────
-// Runs on a timer while the page is visible. Never touches `loading` so
-// existing content stays on screen while the fetch is in flight.
-
-let _refreshTimer: ReturnType<typeof setInterval> | null = null
+// Never touches `loading` so existing content stays on screen while fetching.
 
 function _dateRange() {
   const today = new Date()
@@ -173,9 +159,10 @@ onMounted(async () => {
   chatInputRef.value?.focus();
   loadProjects();
 
-  _refreshTimer = setInterval(_backgroundRefresh, 90_000)
 });
 
+useBackgroundRefresh(_backgroundRefresh, 90_000, () => !loading.value);
+
 async function loadProjects() {
   if (!heroProject.value) return;
   const hid = heroProject.value.id;
@@ -239,7 +226,6 @@ onMounted(() => {
 });
 onUnmounted(() => {
   document.removeEventListener("shortcut:focus-chat", onFocusChatShortcut);
-  if (_refreshTimer !== null) clearInterval(_refreshTimer)
 });
 
 const chatStore = useChatStore();