fix(web): a11y escape + initial focus on relay popover

[intendednull]

Why

Relay-signal popover claims aria-haspopup="dialog" but keyboard users got stranded:

• No Escape close. Global stack in keybindings::install does not know about this popover — Escape was a no-op while focus was inside.
• No initial focus. Tab walked past the dialog content into page chrome.

Issue title says "focus trap" but body says non-modal. Trapping focus on a non-modal popover is wrong — would break Tab-out. Seed focus instead, mark aria-modal="false".

Fix

crates/web/src/components/relay_signal_button.rs:

• Local on:keydown on .relay-popover — Escape sets open=false, stops propagation so global handler does not double-fire.
• Effect on open transition pulls focus to settings-link button via NodeRef + request_animation_frame (same pattern as ConfirmDialog).
• aria-modal="false" + tabindex="-1" on the dialog so it can take focus and AT knows it is non-modal.

No focus trap — Tab continues out of the popover normally.

Verify

• cargo fmt --check clean
• cargo clippy --workspace --all-targets -- -D warnings clean
• cargo check -p willow-web --tests --target wasm32-unknown-unknown clean (Firefox + geckodriver not installed in this env, so wasm-pack browser run is deferred to CI; compile coverage proves the new tests build)
• cargo test -p willow-web --lib 72 passed
• New wasm-pack tests in crates/web/tests/browser.rs:
  • relay_signal_button_escape_closes_popover — opens popover, dispatches Escape keydown, asserts popover unmounts + aria-expanded="false"
  • relay_signal_button_focuses_settings_link_on_open — opens popover, waits one frame, asserts document.activeElement is the settings-link button
  • relay_signal_button_popover_is_non_modal — asserts aria-modal="false" + role="dialog"

Closes #352

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