perf: 16x larger chunks + native backpressure for faster transfers

- CHUNK_SIZE: 16 KB → 256 KB (16x fewer send calls)
- MAX_BUFFER: 1 MB → 4 MB (more data in flight)
- waitForDrain: polling setTimeout → native bufferedamountlow event

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/src/protocol.ts

@@ -164,8 +164,8 @@ export interface PushPayload {
 
 // ── Constants ──
 
-export const CHUNK_SIZE = 16 * 1024; // 16 KB
-export const MAX_BUFFER = 1024 * 1024; // 1 MB backpressure threshold
+export const CHUNK_SIZE = 256 * 1024; // 256 KB
+export const MAX_BUFFER = 4 * 1024 * 1024; // 4 MB backpressure threshold
 
 export const SHORT_CODE_ALPHABET = "abcdefghjkmnpqrstuvwxyz23456789";
 export const SHORT_CODE_LENGTH = 3;

web/src/lib/fileTransfer.ts

@@ -89,15 +89,16 @@ export async function sendFile(
 
 function waitForDrain(peer: SimplePeer.Instance): Promise<void> {
   return new Promise((resolve) => {
-    const check = () => {
-      const channel = (peer as any)._channel as RTCDataChannel | undefined;
-      if (!channel || channel.bufferedAmount < MAX_BUFFER) {
-        resolve();
-      } else {
-        setTimeout(check, 10);
-      }
+    const channel = (peer as any)._channel as RTCDataChannel | undefined;
+    if (!channel || channel.bufferedAmount < MAX_BUFFER) {
+      resolve();
+      return;
+    }
+    channel.bufferedAmountLowThreshold = MAX_BUFFER / 2;
+    channel.onbufferedamountlow = () => {
+      channel.onbufferedamountlow = null;
+      resolve();
     };
-    check();
   });
 }