Anchor IME cursor area at the text input caret

[nicoburns]

Summary

On macOS the IME candidate window appeared at the window's top-left instead of next to the caret. Two causes:

1. blitz-shell: the IME was enabled with ImeCapabilities::new() (no cursor_area capability), so winit discarded every set_ime_cursor_area update (warn!("discarding IME cursor area update without capability enabled.") in winit-appkit) and firstRectForCharacterRange returned the view origin. BlitzShellProvider::set_ime_enabled(true) now requests ImeCapabilities::new().with_cursor_area(). Winit requires an initial cursor area in the enable request (ImeEnableRequest::new returns None otherwise), so the provider caches the last reported area in an ime_cursor_area: Mutex<(LogicalPosition, LogicalSize)> and passes it on enable.

2. blitz-dom: the reported area was the input's whole content box. Node::ime_cursor_area() now returns the caret rect from editor.cursor_geometry(1.5), converted from Parley's scaled pixels to CSS pixels and offset by the same vertical-centering and scroll_offset adjustments the painter applies, so the IME anchor follows the caret as the user types (sync_ime_cursor_area in resolve() re-reports whenever it changes). Falls back to the content box if no text layout exists yet.

Stacked on #843 (needs ime_cursor_area/sync_ime_cursor_area); base will be main once that merges. The unit test from #843 is extended to check the caret-shaped area and that it moves right after typing.

Found during the macOS smoke test of #614 (pre-existing on main).

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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