feat: add dark mode UI and SQLite as alternate database backend

Dark mode: CSS custom properties + .dark class toggled via Redux/localStorage,
with a theme toggle in the Header and Settings page. Foundation didn't exist
yet despite prior assumption, so it was built from scratch.

SQLite: new DB_ENGINE=mongo|sqlite env var selects the metadata backend at
runtime (Mongo stays default, no behavior change unless opted in). File/thumbnail
bytes continue to use the existing fs/S3 storage path. Implemented via a shared
DB interface (dbTypes.ts) with parallel mongoDB/ and sqliteDB/ implementations,
selected through dbFactory.ts, with JWT/encryption logic extracted into
userCrypto.ts so both engines share it.

tests/utils/db-setup.js

@@ -19,6 +19,8 @@ const envFileFix = (env) => {
   env.root = process.env.ROOT;
   env.url = process.env.URL;
   env.mongoURL = process.env.MONGODB_URL;
+  env.dbEngine = process.env.DB_ENGINE || "mongo";
+  env.sqliteDBPath = process.env.SQLITE_DB_PATH || "./data/mydrive.db";
   env.dbType = process.env.DB_TYPE;
   env.fsDirectory = process.env.FS_DIRECTORY;
   env.s3ID = process.env.S3_ID;