fix(electron): prevent unified migration from reverting data dir changes

The unified directory migration (needsUnifiedDirMigration) would
re-trigger after a user changed their data directory, overwriting
config.toml back to the old Electron path.

Root cause: no persistent flag tracking whether the migration had
already completed. If old databases remained (cleanup failure or
timing), it would re-detect and re-run.

Fix:
- Add `electron_migration_done` flag to config.toml [data] section
- Set flag after migrateToUnifiedDirs succeeds and in setCustomDataDir
- Check flag at start of needsUnifiedDirMigration to skip if done
- Add "migrate to default" button when data is at non-default path
- Fix NLP directory path: move from userDataDir to systemDir (~/.chatlab/nlp)

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/cli/src/http/routes/nlp.ts

@@ -35,7 +35,7 @@ function ensureDb(dbManager: DatabaseManager, sessionId: string) {
 export function registerNlpRoutes(server: FastifyInstance, dbManager: DatabaseManager): void {
   const pathProvider = (dbManager as any)['pathProvider']
   if (pathProvider) {
-    const nlpDir = path.join(pathProvider.getUserDataDir(), 'nlp')
+    const nlpDir = path.join(pathProvider.getSystemDir(), 'nlp')
     initNlpDir(nlpDir)
     ensureDefaultDict(nlpDir).catch((err) => console.warn('[NLP] Auto-download zh-CN dict failed:', err))
   }
@@ -81,5 +81,5 @@ export function registerNlpRoutes(server: FastifyInstance, dbManager: DatabaseMa
 
 function resolveNlpDir(dbManager: DatabaseManager): string {
   const pathProvider = (dbManager as any)['pathProvider']
-  return path.join(pathProvider.getUserDataDir(), 'nlp')
+  return path.join(pathProvider.getSystemDir(), 'nlp')
 }