Implement :focus-visible

[nicoburns]

Summary

Wires up :focus-visible matching to the FOCUSRING element-state bit, and makes that bit reflect how focus was acquired instead of being set unconditionally on every focus.

• New FocusSource { Keyboard, Pointer, Script } and BaseDocument::set_focus_to_with_source(id, source); set_focus_to(id) keeps its signature and now means Script.
• Ring heuristics: Keyboard/Script focus → focus ring; Pointer focus → ring only for keyboard-input elements (text inputs). Refocusing an already-focused node with a different source updates the ring bit (snapshotted, so it restyles correctly).
• Node::focus(shell_provider, focus_visible) sets FOCUS always and FOCUSRING conditionally; match_non_ts_pseudo_class now matches FocusVisible against FOCUSRING (previously hardcoded false).
• Callers: Tab traversal (focus_next_node/focus_prev_node) → Keyboard; pointer/click handlers (text input, checkbox, radio, summary, subdoc, custom widget) → Pointer; autofocus and dioxus-native-dom script focus → Script.
• UA stylesheet: input:focus, textarea:focus { outline: ... } replaced with a generic :focus-visible { outline: 2px solid #4D90FE; }, matching browser UA behavior — keyboard-focused buttons/links etc. now get a focus ring, mouse-clicked ones don't.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/65287500bfb04b2589b9fc6a08bc2d14
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/65287500bfb04b2589b9fc6a08bc2d14?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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