feat: Implement dark/light mode toggle and address linting issues

This commit introduces a functional dark/light mode toggle by integrating
`next-themes` with the application. It configures `ThemeProvider` in
`src/frontend.tsx` and sets `darkMode: 'class'` in `tailwind.config.js`.

Additionally, this commit resolves several linting and parsing issues
found in `src/index.ts`.

This commit also includes other pre-existing modifications and new
components within the project's working directory that were not
specifically part of the dark mode or linting tasks.

src/routes/dashboard/index.ts

@@ -0,0 +1,5 @@
+import { createFileRoute } from "@tanstack/react-router";
+import { DashboardContent } from "../../app/components/dashboard-content";
+export const Route = createFileRoute("/dashboard/")({
+	component: DashboardContent,
+});