feat: preview Agent HTML in conversations and unblock long replies

Agent replies that produced HTML could only be read as source, so users had
to copy the markup into an external file to see the result. Completed Agent
and Subagent output now renders complete HTML documents, single-root HTML
replies, and `html` / `htm` code blocks as an isolated static preview in
place. Each preview keeps a persistent notice that scripts and network are
disabled and offers the same icon actions as Mermaid diagrams: a source
toggle and a full-screen preview. The new "render HTML in conversations"
switch under Platform Features / General is on by default, applies to the
open conversation immediately, and falls back to HTML source when disabled.
Previews cannot run scripts, reach the network, submit forms, open windows,
or load remote media; author stylesheets and inline styles are preserved so
the intended layout survives, and remote resource loading is refused by the
preview CSP instead.

Direct-model replies no longer fail once they grow large. The byte caps on
streaming responses, single stream blocks, non-streaming replies, model tool
arguments, tool request context, aggregate tool results, and accumulated
answer text are removed, so users stop seeing "模型接口流式响应超过安全限制"
on long answers. Response length is now governed by the request timeout,
cancellation, and context compression. Image generation responses keep their
decodable-image validation.

A message sent after a failed or interrupted run stayed in the pending queue
instead of running. The conversation is now released whenever a run reaches
any terminal state, not only when the renderer reports readiness again, and a
dispatched queue item whose run never starts releases its reservation and
returns to pending. Later messages dispatch without extra user action.

Release note: Agent 回复中的 HTML 现在可直接在会话内预览，支持查看源码和全屏预览，默认开启并可在“平台功能 / 通用设置”关闭；直连模型的长回复不再因大小限制中断；上一次运行失败或被中断后，新发送的消息会立即执行而不再排队等待。

FEATURES.md

@@ -32,9 +32,13 @@ otherwise.
   Host heading and only the remote path on each project row.
 - [x] **File, screenshot, window, and clipboard context**: Added to model
   context only after explicit user selection.
-- [x] **Rich responses**: Supports GitHub Flavored Markdown, LaTeX math, and
-  constrained Mermaid diagrams. Large diagrams can be zoomed, panned, or
-  viewed as source, and failed renders preserve the original diagram code.
+- [x] **Rich responses**: Supports GitHub Flavored Markdown, LaTeX math,
+  constrained Mermaid diagrams, and static in-conversation HTML previews.
+  Complete HTML and HTML code blocks can be previewed in place after an Agent
+  reply finishes, with icon actions to reveal the source or open a full-screen
+  preview. The feature is on by default and can be disabled under Platform
+  Features / General; previews cannot run scripts, access the network, submit
+  forms, or open windows.
 - [x] **AI response and full-conversation copy**: Completed AI responses expose
   a bottom action that copies Markdown source without reasoning, tool logs, or
   citation metadata. Full-conversation copy uses the same validated