feat(story-2.4): home screen device list with FrameCard component

- FrameCard: large (single device, 5:3 preview + Add Photo CTA) and
  compact (52px thumb + name + count + icon pill) variants; WCAG-
  compliant offline/sync-fail status (color + text, never color alone)
- devices Pinia store: fetchDevices() → GET /api/devices
- HomeView: 0 devices → dashed empty-state card; 1 device → large
  FrameCard; 2+ → compact stack; add-photo wired (Epic 3 stub)
- Fix Device type: rotationInterval → rotationIntervalHours to match API

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/devices.ts

@@ -0,0 +1,25 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import type { Device } from '@/types'
+
+export const useDevicesStore = defineStore('devices', () => {
+  const devices = ref<Device[]>([])
+  const loading = ref(false)
+  const error = ref<string | null>(null)
+
+  async function fetchDevices() {
+    loading.value = true
+    error.value = null
+    try {
+      const res = await fetch('/api/devices')
+      if (!res.ok) throw new Error('Failed to load devices')
+      devices.value = await res.json()
+    } catch (e) {
+      error.value = e instanceof Error ? e.message : 'Unknown error'
+    } finally {
+      loading.value = false
+    }
+  }
+
+  return { devices, loading, error, fetchDevices }
+})