feat(PBI-76): bridge runs one-shot localStorage migration

After hydrate, scans legacy localStorage keys via buildMigrationPatch
and, if any data is found, pushes one bulk patch to the server,
applies it locally, then removes the legacy keys. Demo accounts skip
the migration entirely. Cancellable on unmount to avoid setState on
unmounted component.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

components/shared/user-settings-bridge.tsx

@@ -1,8 +1,13 @@
 'use client'
 
 import { useEffect } from 'react'
+import { updateUserSettingsAction } from '@/actions/user-settings'
 import { useUserSettingsStore } from '@/stores/user-settings/store'
 import type { UserSettings } from '@/lib/user-settings'
+import {
+  buildMigrationPatch,
+  clearLegacyLocalStorage,
+} from '@/lib/user-settings-migration'
 
 interface Props {
   initial: UserSettings
@@ -23,6 +28,29 @@ export function UserSettingsBridge({ initial, isDemo }: Props) {
     hydrate(initial, isDemo)
   }, [hydrate, initial, isDemo])
 
+  // One-shot migration: read legacy localStorage prefs, push to server, clear.
+  // Idempotent via marker; demo accounts skip (no server-write).
+  useEffect(() => {
+    if (isDemo) return
+    const result = buildMigrationPatch()
+    if (!result.hasData) {
+      clearLegacyLocalStorage([])
+      return
+    }
+    let cancelled = false
+    void (async () => {
+      const res = await updateUserSettingsAction(result.patch)
+      if (cancelled) return
+      if ('success' in res && res.success) {
+        applyServerPatch(result.patch)
+        clearLegacyLocalStorage(result.legacyKeys)
+      }
+    })()
+    return () => {
+      cancelled = true
+    }
+  }, [isDemo, applyServerPatch])
+
   useEffect(() => {
     if (isDemo) return
     const es = new EventSource('/api/realtime/user-settings')