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.

backend/utils/userUpdateCheck.ts

@@ -1,17 +1,12 @@
-import User, { UserInterface } from "../models/user-model";
-import mongoose from "mongoose";
-import { Request, Response, NextFunction } from "express";
+import { Response } from "express";
 import NotFoundError from "./NotFoundError";
 import { createLoginCookie } from "../cookies/create-cookies";
+import { getUserDB } from "../db/dbFactory";
 
-// interface RequestType extends Request {
-//     user?: userAccessType,
-//     token?: string,
-//     encryptedToken?: string,
-// }
+const userDB = getUserDB();
 
 type userAccessType = {
-  _id: mongoose.Types.ObjectId;
+  _id: any;
   emailVerified: boolean;
   email: string;
   botChecked: boolean;
@@ -19,10 +14,10 @@ type userAccessType = {
 
 const userUpdateCheck = async (
   res: Response,
-  id: mongoose.Types.ObjectId,
+  id: string,
   uuid: string | undefined
 ) => {
-  const updatedUser = await User.findById(id);
+  const updatedUser = await userDB.getUserInfo(id);
 
   if (!updatedUser) throw new NotFoundError("Cannot find updated user auth");