fix(web): wire active-row a11y on search listbox

[intendednull]

Why

<ResultsList> pass every row selected=Signal::derive(|| false). No roving tabindex, no aria-activedescendant on input. Keyboard user not see which row Enter target. Screen reader never see selected option in listbox. Spec say listbox MUST mark active option — this regress a11y baseline. Issue tag medium severity, [GEN-01].

Fix

Track active_index: ReadSignal<usize> in SearchUiState. Default 0. Reset to 0 whenever result set or scope change (Effect in surface) — stale index never outlive its data.

Per-row selected derive from flat (in-display-order) index match: Signal::derive(move || active_index.get() == flat). flat = group_offset + intra_group_idx. Same flat_ordered() helper used by input so input + listbox agree on what "row N" mean.

Input keydown handle Up/Down/Home/End to move active_index. Up/Down wrap (listbox-pattern default; match command palette behavior). aria-activedescendant on input point at id="search-row-{message_id}" of active row, omitted (Option::None) when results empty per WAI-ARIA.

Enter still push to recents — not change in this PR. Enter-activate-active-row deserve separate change (touch surface callback wiring); track as follow-up.

Runner-up rejected: track active-row by message_id instead of flat index. Survive reorder better but cost extra lookup on every keystroke + complicate Home/End semantic. Flat index simpler, reset-on-change keep stale-pointer risk bounded.

Verify

• cargo clippy --workspace --all-targets -- -D warnings — clean.
• cargo fmt --check — clean.
• cargo check -p willow-web --target wasm32-unknown-unknown — clean.
• cargo test -p willow-web --lib — 68 pass.
• New wasm-pack browser tests in crates/web/tests/browser.rs (phase_2e_search_active_row mod) — three test:
  • active row carry aria-selected="true", others false;
  • moving active_index swap selection reactively;
  • flat index walk pick right row under BTreeMap-grouped AllGrovesAndLetters scope (regression guard for "indexes raw vec, not display order" bug).
• wasm-pack / Firefox / geckodriver not present in this env — browser tests not run locally. Compile-time verify via clippy --all-targets (cover the wasm test target).

Closes #344

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