docs(ui): clarify UIElement value is read-only

[onthebed]

This pull request was authored by a coding agent.

The shared UIElement docs described value without noting that it is read-only, even though the runtime rejects direct assignment with a RuntimeError and points users to mo.state().

This updates the UIElement class and property docstrings to say that value reflects frontend state, cannot be assigned directly, and that mo.state() is the supported way to imperatively drive UI state.

Verification:

• reproduced the current runtime behavior in a local venv (x.value = "hello" raises RuntimeError: Setting the value of a UIElement is not allowed...)
• confirmed the updated docstrings through Python introspection after the edit
• attempted mkdocs build --strict -q, but the local docs toolchain failed before rendering due to a mkdocstrings/mkdocs config mismatch in this ad-hoc environment (PythonConfig.__init__() got an unexpected keyword argument 'import')

Fixes #9203

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Apr 25, 2026 0:30am

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