feat: add recentFlows localStorage utility for command palette empty state

Tracks recently visited flows (capped at 10) with deduplication by id,
surfaced in command palette when query is empty.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/recentFlows.ts

@@ -0,0 +1,40 @@
+/**
+ * localStorage utility for tracking recently visited flows.
+ */
+
+const STORAGE_KEY = 'rf_recent_flows'
+const MAX_ENTRIES = 10
+
+export interface RecentFlow {
+  id: string
+  name: string
+  tree_type: string
+  timestamp: number
+}
+
+export function getRecentFlows(limit = 5): RecentFlow[] {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY)
+    if (!raw) return []
+    const parsed = JSON.parse(raw) as RecentFlow[]
+    return Array.isArray(parsed) ? parsed.slice(0, limit) : []
+  } catch {
+    return []
+  }
+}
+
+export function addRecentFlow(flow: Omit<RecentFlow, 'timestamp'>): void {
+  try {
+    const existing = getRecentFlows(MAX_ENTRIES)
+    // Deduplicate by id — remove any existing entry with the same id
+    const deduped = existing.filter(f => f.id !== flow.id)
+    // Add to front with current timestamp
+    const updated: RecentFlow[] = [
+      { ...flow, timestamp: Date.now() },
+      ...deduped,
+    ].slice(0, MAX_ENTRIES)
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(updated))
+  } catch {
+    // Silently ignore localStorage errors (private browsing, quota exceeded)
+  }
+}