Add SVG assets and TypeScript configuration

- Created a new SVG for the Open Graph image (og-image.svg) with a gradient background and text elements.
- Added Vercel logo SVG (vercel.svg) for deployment branding.
- Introduced a window icon SVG (window.svg) for UI representation.
- Initialized TypeScript configuration file (tsconfig.json) with strict settings and module resolution for a React project.

app/hooks/useTheme.ts

@@ -0,0 +1,28 @@
+"use client";
+
+import { useState, useEffect } from "react";
+import { Theme } from "../types";
+
+export function useTheme() {
+  const [theme, setTheme] = useState<Theme>("light");
+
+  useEffect(() => {
+    const saved = localStorage.getItem("blocnote-theme") as Theme | null;
+    if (saved) {
+      // Reading persisted theme must happen on client after mount.
+      // eslint-disable-next-line react-hooks/set-state-in-effect
+      setTheme(saved);
+    } else if (window.matchMedia("(prefers-color-scheme: dark)").matches) {
+      setTheme("dark");
+    }
+  }, []);
+
+  useEffect(() => {
+    document.documentElement.setAttribute("data-theme", theme);
+    localStorage.setItem("blocnote-theme", theme);
+  }, [theme]);
+
+  const toggleTheme = () => setTheme((t) => (t === "light" ? "dark" : "light"));
+
+  return { theme, toggleTheme };
+}