Raise avatar cap 64 to 128 KiB, pipeline checks encoded size

64 KiB rejected normal 128px PNGs. 128 KiB fits any 128x128 PNG
(raw <= 64 KiB, worst-case base64 ~95 KiB chars) while still
bounding abuse. PNG kept for transparency. Pipeline now measures
the encoded base64 against RelayConstants (was checking the wrong
variable with its own 512 KiB limit) and drops dead crop code.
Server policy, still v4. Trade noted: avatars ride the 1 Hz room
broadcast, uplink scales with cap; hash-reference follow-up later.

apps/desktop/lib/src/widgets/avatar_pipeline.dart

@@ -3,6 +3,7 @@ import 'dart:io';
 import 'dart:typed_data';
 
 import 'package:image/image.dart' as img;
+import 'package:sync_relay_core/core.dart';
 
 Future<String> encodeAvatar(String path) async {
   final trimmed = path.trim();
@@ -24,22 +25,18 @@ Future<String> encodeAvatar(String path) async {
   if (decoded == null) {
     throw const FormatException('could not read that image — pick a PNG, JPG or WebP photo');
   }
-  // final side = decoded.width < decoded.height ? decoded.width : decoded.height;
-  // final x = (decoded.width - side) ~/ 2;
-  // final y = (decoded.height - side) ~/ 2;
-  // final cropped = img.copyCrop(decoded, x: x, y: y, width: side, height: side);
   final resized = img.copyResize(
     decoded,
     width: 128,
     height: 128,
     maintainAspect: true,
-    // backgroundColor: img.ColorFloat16.rgba(0, 0, 0, 0),
     backgroundColor: img.PixelUndefined(),
     interpolation: img.Interpolation.cubic,
   );
   final png = img.encodePng(resized);
-  if (resized.length > 512 * 1024) {
+  final b64 = base64Encode(png);
+  if (b64.length > RelayConstants.maxAvatarBytes) {
     throw const FormatException('that photo is still too large after shrinking — pick a smaller one');
   }
-  return base64Encode(png);
+  return b64;
 }