feat(chat): image specialist session image fallback and WS UX

- Resolve legacy multimodal inputs from session history when the user sends
  text-only (assistant images first, then user uploads); optional env toggles.
- Support relay_pointer image resolution in legacy lane collector.
- Align uvicorn WebSocket frame limit with MAX_PAYLOAD_BYTES; document
  OCLAW_UVICORN_WS_MAX_SIZE and compatible-mode env knobs.
- Admin chat: encode attachments after bubble preview; FileReader-based
  base64 for large files.
- Tests and docs for new behavior.

Co-authored-by: Cursor <cursoragent@cursor.com>

_local/system.env.example

@@ -29,6 +29,11 @@
 AIA_ASSISTANT_GATEWAY_HOST=0.0.0.0
 AIA_ASSISTANT_GATEWAY_PORT=8787
 
+# OCLAW_UVICORN_WS_MAX_SIZE    uvicorn WebSocket 单帧最大字节；默认与 hello 里 maxPayload 一致（约 25MB）。
+# 不设时 python -m oclaw... 仍用旧 uvicorn 默认 16MB，多图 base64 一条 chat.send 易超限被断连，前端报 ws_closed。
+# 若用命令行启动 uvicorn，请自行加：--ws-max-size 26214400（或与 MAX_PAYLOAD_BYTES 一致）。
+# OCLAW_UVICORN_WS_MAX_SIZE=
+
 # AIA_PREWARM_INTERVAL_SECONDS    后台预热任务周期（秒），默认 600，合法范围代码内 clamp。
 # 【前端】无。
 AIA_PREWARM_INTERVAL_SECONDS=600
@@ -287,6 +292,14 @@ AIA_IMAGE_EXPERT_MODEL=
 AIA_IMAGE_EXPERT_BASE_URL=
 AIA_IMAGE_EXPERT_API_KEY=
 AIA_IMAGE_EXPERT_CHAT_ENDPOINT=
+# compatible-mode defaults to OpenAI vision blocks (type + image_url/text). Set to 1 only if your gateway
+# requires DashScope doc shape without "type" on each content part (may break OpenAI-compatible gateways).
+# AIA_IMAGE_EXPERT_COMPAT_USE_DASHSCOPE_NATIVE_BLOCKS=
+# Max input images per turn (1–12; default 8).
+# AIA_IMAGE_EXPERT_MAX_INPUT_IMAGES=
+# Multi-turn image specialist: inherit images from assistant/user history when this turn has no attachment (0=off).
+# AIA_IMAGE_SPECIALIST_SESSION_IMAGE_FALLBACK=
+# AIA_IMAGE_SPECIALIST_FALLBACK_SCAN_MESSAGES=
 
 # Optional; image expert top-level kwargs (parity with DashScope multimodal-gen samples)
 DASHSCOPE_IMAGE_STREAM=