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/BriefingView.vue

@@ -1,5 +1,6 @@
 <script setup lang="ts">
-import { ref, computed, onMounted, onUnmounted, watch } from 'vue'
+import { ref, computed, onMounted, watch } from 'vue'
+import { useBackgroundRefresh } from '@/composables/useBackgroundRefresh'
 import { useChatStore } from '@/stores/chat'
 import ChatMessage from '@/components/ChatMessage.vue'
 import WeatherCard from '@/components/WeatherCard.vue'
@@ -190,10 +191,7 @@ function toMsg(m: BriefingMessage): Message {
 }
 
 // ─── Background refresh (no-flicker) ─────────────────────────────────────────
-let _refreshTimer: ReturnType<typeof setInterval> | null = null
-
 async function _backgroundRefreshMessages() {
-  if (document.hidden || chatStore.streaming || !isToday.value || !todayConvId.value) return
   try {
     const today = await getBriefingToday()
     if (!today) return
@@ -206,14 +204,15 @@ async function _backgroundRefreshMessages() {
   } catch { /* silent — don't disturb the UI on network hiccup */ }
 }
 
+useBackgroundRefresh(
+  _backgroundRefreshMessages,
+  60_000,
+  () => !chatStore.streaming && isToday.value && !!todayConvId.value,
+)
+
 onMounted(async () => {
   await checkSetup()
   if (!showWizard.value) await loadAll()
-  _refreshTimer = setInterval(_backgroundRefreshMessages, 60_000)
-})
-
-onUnmounted(() => {
-  if (_refreshTimer !== null) clearInterval(_refreshTimer)
 })
 </script>