feat: import SQLite migration dumps through the PostgreSQL panel restore

The SQLite panel's Download Migration produces a portable SQL text dump
advertised as seeding a PostgreSQL panel, but the PostgreSQL Restore only
accepted pg_dump custom archives, so the migration file was rejected with
'Invalid file' even though the upload picker asked for .dump. importDB now
sniffs the upload header: PGDMP archives keep the pg_restore path, while
raw SQLite databases (.db) and SQL text migration dumps are rebuilt,
integrity-checked, and copied into PostgreSQL with the same MigrateData
engine as 'x-ui migrate-db --dsn'. The restore picker accepts .dump/.db on
PostgreSQL and the backup modal texts describe the accepted formats in
every locale.

frontend/src/pages/index/BackupModal.tsx

@@ -32,7 +32,7 @@ export default function BackupModal({ open, basePath: _basePath, onClose, onBusy
   function importDb() {
     const fileInput = document.createElement('input');
     fileInput.type = 'file';
-    fileInput.accept = isPostgres ? '.dump' : '.db';
+    fileInput.accept = isPostgres ? '.dump,.db' : '.db';
     fileInput.addEventListener('change', async (e) => {
       const dbFile = (e.target as HTMLInputElement).files?.[0];
       if (!dbFile) return;