Add kick player and leave game functionality

- Host can kick players from lobby (removes from game, clears presenter if needed)
- Client can voluntarily leave game
- Fix browser-compatible base64 decoding for document upload (atob vs Buffer)

services/geminiService.ts

@@ -130,11 +130,20 @@ function transformToQuiz(data: any): Quiz {
 }
 
 async function uploadNativeDocument(ai: GoogleGenAI, doc: ProcessedDocument): Promise<{ uri: string; mimeType: string }> {
-  const buffer = typeof doc.content === 'string'
-    ? Buffer.from(doc.content, 'base64')
-    : doc.content;
+  let data: ArrayBuffer;
   
-  const blob = new Blob([buffer], { type: doc.mimeType });
+  if (typeof doc.content === 'string') {
+    const binaryString = atob(doc.content);
+    const bytes = new Uint8Array(binaryString.length);
+    for (let i = 0; i < binaryString.length; i++) {
+      bytes[i] = binaryString.charCodeAt(i);
+    }
+    data = bytes.buffer as ArrayBuffer;
+  } else {
+    data = doc.content;
+  }
+  
+  const blob = new Blob([data], { type: doc.mimeType });
   
   const uploadedFile = await ai.files.upload({
     file: blob,