Populate AccessKit node bounds from layout

[nicoburns]

Summary

AccessKit nodes built by BaseDocument::build_accessibility_tree never had bounds set, so every node reported a 0×0 frame (macOS AX dump showed all content at (0,1200) 0x0). VoiceOver's cursor navigation skips zero-size nodes, so only focus-driven announcements worked. This is platform-independent (AT-SPI/UIA use the same property); the macOS set_window_bounds no-op in accesskit_xplat is not the cause — the macOS adapter derives the frame from the NSView and only needs per-node bounds.

build_accessibility_node now sets:

builder.set_bounds(self.accessibility_bounds(node))
// = get_client_bounding_rect(node_or_parent) * viewport.scale

• Coordinates are physical pixels relative to the viewport origin (what accesskit_consumer expects; accesskit_macos::to_ns_rect divides by backingScaleFactor).
• get_client_bounding_rect already handles inline (non-atomic) elements via per-line fragment rects and subtracts viewport_scroll.
• Text nodes have no layout box; they use their parent element's rect. Comment nodes are skipped (layout_data() panics for them).

Adds a unit test checking element/text bounds at scale 2.0.

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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