Initial commit: modern React video player library

Add all source files for a feature-rich, reusable video player built with React, TypeScript, and Vite. Includes core components, context, hooks, utilities, styles, demo app, and configuration files.

src/components/controls/FullscreenButton.tsx

@@ -0,0 +1,23 @@
+import React from 'react'
+import { usePlayerContext } from '../../contexts/PlayerContext'
+import { FullscreenIcon, FullscreenExitIcon } from '../../icons'
+import './ControlButton.css'
+
+export const FullscreenButton: React.FC = () => {
+  const { videoState, toggleFullscreen } = usePlayerContext()
+
+  return (
+    <button
+      className="control-button fullscreen-button"
+      onClick={toggleFullscreen}
+      aria-label={videoState.fullscreen ? 'Exit fullscreen' : 'Enter fullscreen'}
+      title={videoState.fullscreen ? 'Exit fullscreen (F)' : 'Enter fullscreen (F)'}
+    >
+      {videoState.fullscreen ? (
+        <FullscreenExitIcon size={24} color="var(--player-text)" />
+      ) : (
+        <FullscreenIcon size={24} color="var(--player-text)" />
+      )}
+    </button>
+  )
+}