feat: implement global app state store and custom MUI theme configuration

frontend/src/store/useAppStore.ts

@@ -0,0 +1,19 @@
+import { create } from 'zustand';
+
+interface AppState {
+  darkMode: boolean;
+  activeDatabase: string | null;
+  activeTable: string | null;
+  toggleDarkMode: () => void;
+  setActiveDatabase: (db: string | null) => void;
+  setActiveTable: (table: string | null) => void;
+}
+
+export const useAppStore = create<AppState>((set) => ({
+  darkMode: true,
+  activeDatabase: null,
+  activeTable: null,
+  toggleDarkMode: () => set((state) => ({ darkMode: !state.darkMode })),
+  setActiveDatabase: (db) => set({ activeDatabase: db, activeTable: null }),
+  setActiveTable: (table) => set({ activeTable: table }),
+}));