feat: add migration wizard for flat vault

On vault load, detects files in non-protected subfolders via
vault_health_check. Shows an amber banner offering to flatten them
to the vault root. After migration, reloads the vault automatically.
The banner can be dismissed without migrating.

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

src/hooks/useFlatVaultMigration.ts

@@ -0,0 +1,71 @@
+import { useState, useEffect, useCallback } from 'react'
+import { invoke } from '@tauri-apps/api/core'
+import { isTauri } from '../mock-tauri'
+
+interface HealthReport {
+  stray_files: string[]
+  title_mismatches: { path: string; filename: string; title: string; expected_filename: string }[]
+}
+
+interface FlatVaultMigration {
+  /** True if stray files were detected in non-protected subfolders. */
+  needsMigration: boolean
+  /** List of stray file paths (relative to vault root). */
+  strayFiles: string[]
+  /** Dismiss the migration prompt without migrating. */
+  dismiss: () => void
+  /** Run flatten_vault and reload. Returns the count of files moved. */
+  migrate: () => Promise<number>
+  /** True while migration is running. */
+  isMigrating: boolean
+}
+
+/**
+ * Detects if the vault has files in non-protected subfolders and offers
+ * to flatten them to the vault root. Runs once on vault load.
+ */
+export function useFlatVaultMigration(
+  vaultPath: string,
+  entriesLoaded: boolean,
+  reloadVault: () => Promise<void>,
+): FlatVaultMigration {
+  const [strayFiles, setStrayFiles] = useState<string[]>([])
+  const [dismissed, setDismissed] = useState(false)
+  const [isMigrating, setIsMigrating] = useState(false)
+
+  useEffect(() => {
+    if (!entriesLoaded || !vaultPath || !isTauri()) return
+    let cancelled = false
+    invoke<HealthReport>('vault_health_check', { vaultPath })
+      .then((report) => {
+        if (!cancelled && report.stray_files.length > 0) {
+          setStrayFiles(report.stray_files)
+        }
+      })
+      .catch(() => { /* non-critical */ })
+    return () => { cancelled = true }
+  }, [vaultPath, entriesLoaded])
+
+  const dismiss = useCallback(() => setDismissed(true), [])
+
+  const migrate = useCallback(async () => {
+    setIsMigrating(true)
+    try {
+      const count = await invoke<number>('flatten_vault', { vaultPath })
+      setStrayFiles([])
+      setDismissed(true)
+      await reloadVault()
+      return count
+    } finally {
+      setIsMigrating(false)
+    }
+  }, [vaultPath, reloadVault])
+
+  return {
+    needsMigration: strayFiles.length > 0 && !dismissed,
+    strayFiles,
+    dismiss,
+    migrate,
+    isMigrating,
+  }
+}