Add initial project files

packages/hooks/src/useIsMobile/Readme.md

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+# useIsMobile
+
+Returns `true` when the viewport width is below a configurable breakpoint, and re-evaluates reactively on resize.
+
+## Signature
+
+```ts
+function useIsMobile(mobileBreakpoint?: number): boolean;
+```
+
+| Parameter          | Type     | Default | Description                                                  |
+| ------------------ | -------- | ------- | ------------------------------------------------------------ |
+| `mobileBreakpoint` | `number` | `1024`  | Viewport width (px) at which the device is considered mobile |
+
+## Usage
+
+```tsx
+function Navbar() {
+  const isMobile = useIsMobile();
+
+  return isMobile ? <MobileNav /> : <DesktopNav />;
+}
+```
+
+```tsx
+// Custom breakpoint
+const isTablet = useIsMobile(768);
+```
+
+## Notes
+
+- Uses `window.matchMedia` for the resize listener (more efficient than `resize` event polling).
+- Returns `false` during SSR (initial state is `undefined`, coerced via `!!`).
+- Client-only (`'use client'`).