Avatar cap to 256 KiB; idle warn only while ready

Private-server headroom over any pipeline PNG. The 8-min
auto-unready warning now mirrors the 10-min action guard: no toast
when self isn't ready, since there's nothing to lose.

packages/core/lib/src/constants.dart

@@ -15,13 +15,14 @@ abstract class RelayConstants {
   static const int stateHeartbeatSecs = 1;
   static const int pingIntervalSecs = 5;
 
-  // Base64-char count of the avatar string, enforced at hello. Sized so
-  // ANY 128x128 pipeline PNG always fits (raw bitmap <= 64 KiB, worst-case
-  // PNG ~70 KiB raw ~= 95 KiB base64). PNG keeps transparency; JPEG did not.
+  // Base64-char count of the avatar string, enforced at hello. 256 KiB
+  // leaves wide headroom over any 128x128 pipeline PNG (raw bitmap
+  // <= 64 KiB, worst-case base64 ~95 KiB chars) — deliberate over-size
+  // for private-server use. PNG keeps transparency; JPEG did not.
   // Trade: avatars ride in the 1 Hz room broadcast, so worst-case host
   // uplink scales with this cap. Follow-up (not now): send avatar bytes
   // once, reference by hash afterwards.
-  static const int maxAvatarBytes = 128 * 1024;
+  static const int maxAvatarBytes = 256 * 1024;
   static const int maxNameLength = 32;
   static const int maxChatLength = 500;
 }