fix: server-side payload cap + cleaner image abort

Addresses Copilot review on PR #9:

- /api/vision: add MAX_ANNOTATED_BYTES (3 MB) cap on annotatedImageBase64,
  plus an explicit type/non-empty check. Browser annotator resizes to 768
  wide (typically 200-800 KB base64), so 3 MB rejects abusive direct-API
  payloads that would otherwise inflate upstream vision LLM costs.
- annotateClient: replace `img.src = ""` on timeout with removeAttribute
  to avoid the legacy browser behavior of treating empty src as a
  navigation to the current document URL.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/web/app/api/vision/route.ts

@@ -6,6 +6,11 @@ import { loadEngineConfig } from "@/lib/config";
 export const runtime = "nodejs";
 export const maxDuration = 60;
 
+// Browser annotator resizes to 768 wide → typically 200-800 KB base64.
+// 3 MB caps abusive direct-API payloads (which would inflate upstream
+// vision LLM costs) while leaving ~4x headroom for legitimate inputs.
+const MAX_ANNOTATED_BYTES = 3 * 1024 * 1024;
+
 export async function POST(req: Request) {
   let body: VisionRequest;
   try {
@@ -14,12 +19,27 @@ export async function POST(req: Request) {
     return NextResponse.json({ error: "Invalid JSON" }, { status: 400 });
   }
 
-  if (!body.session || !body.annotatedImageBase64) {
+  if (!body.session) {
     return NextResponse.json(
-      { error: "session and annotatedImageBase64 are required" },
+      { error: "session is required" },
       { status: 400 },
     );
   }
+  if (
+    typeof body.annotatedImageBase64 !== "string" ||
+    body.annotatedImageBase64.length === 0
+  ) {
+    return NextResponse.json(
+      { error: "annotatedImageBase64 must be a non-empty string" },
+      { status: 400 },
+    );
+  }
+  if (body.annotatedImageBase64.length > MAX_ANNOTATED_BYTES) {
+    return NextResponse.json(
+      { error: `annotatedImageBase64 exceeds ${MAX_ANNOTATED_BYTES} bytes` },
+      { status: 413 },
+    );
+  }
 
   try {
     const config = loadEngineConfig();