Enable IME for text inputs focussed before first layout

[nicoburns]

Summary

With a CJK input source active, typing into an input that is focussed at startup (autofocus, or onmounted → set_focus) produced raw ASCII until the input was blurred and re-focussed.

Cause: Node::focus decided "is this a text input?" via element_data().text_input_data().is_some(), but TextInputData is only created during layout construction (create_text_editor in layout/construct.rs). Autofocus runs at mutation-flush time, before the first resolve(), so set_ime_enabled(true) was never called.

Changes (blitz-dom):

• ElementData::is_text_input() — tag/type-based check (same set as construct.rs), independent of layout. Node::focus/blur now use it to enable/disable the IME.
• Node::ime_cursor_area() -> Option<(x, y, w, h)> returns None before layout instead of reading a not-yet-computed final_layout.
• BaseDocument::sync_ime_cursor_area() is called at the end of resolve() after layout: it reports the focussed text input's content box to the shell, deduplicated against last_ime_cursor_area so it only fires when the area actually changes (including after the first layout of an autofocussed input, or when the input moves due to relayout).

Adds a unit test with a recording ShellProvider covering focus-before-layout → set_ime_enabled(true), area reported once after resolve, no re-report on unchanged layout, and set_ime_enabled(false) on blur.

Found during the macOS smoke test of #614 (pre-existing on main). On macOS the cursor area is currently still discarded by winit because Blitz enables the IME without the cursor_area capability; that is fixed in a separate PR.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/a737411590c54bbb833aa9167f89ac51
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/a737411590c54bbb833aa9167f89ac51?variant=devin-insiders
Requested by: @nicoburns

WPT results

No changes in test results compared to main.

_{Generated by the WPT workflow.}

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