Web UI: chat column doesn't use full width on wide screens — add a layout/width option

Is your feature request related to a problem?

When running kimi web on a wide-screen monitor, the chat conversation column (messages and the input box) is constrained to a narrow, centered max-width, leaving large empty margins on both sides. There is currently no setting, config key, or CLI flag to adjust this.

Describe the solution you'd like

A setting (in the web Settings page, or config.toml) to control the chat layout width — e.g. a "wide mode" / full-width layout toggle, or a configurable max-width for the conversation column.

Describe alternatives you've considered

• Browser zoom only scales the fixed-width column, it does not widen it.
• Overriding CSS via devtools works but is not persistent.

Additional context

• Version: 0.41.0
• OS: Windows
• Docs checked: no width-related option is documented in the web guide (kimi-code-cli/guides/web.html) or the kimi web command reference.

Start by reading guides/web.html and the kimi web command reference, then trace how the Web UI Settings page and config.toml options are handled. Decide whether the width control belongs in settings, configuration, or both. Done means the chat column can use a user-controlled wider or full-width layout and the option is documented.