Theme rework: polish dark mode and previews

Refine mobile nav layout, improve dark theme UI polish, and update README previews with light/dark variants.

models/User.js

@@ -9,6 +9,8 @@ const userSchema = new mongoose.Schema({
   mustChangePassword: { type: Boolean, default: false },
   tempPasswordIssuedAt: { type: Date, default: null },
   tempPasswordExpiresAt: { type: Date, default: null },
+  // Optional. When unset, UI follows system preference.
+  themePreference: { type: String, enum: ["light", "dark"], default: undefined },
   watchedMovies: [{
     imdb_id: { type: String, required: false },
     watchedDate: { type: Date, default: Date.now }  // Store the watched date