feat(webui): session list ARIA tree semantics + roving tabindex

[edwin-zvs]

Closes #1075.

The web UI session list had TUI-parity keyboard navigation but no accessibility semantics: rows were anonymous divs, selection was communicated by a CSS class only, and no row was ever in the tab order, so a screen reader announced nothing useful and a Tab-only user could never reach the list.

What changed (crates/daemon/assets/index.html)

• role="tree" on the container (aria-label="sessions"). Chose tree over listbox because the list genuinely nests — groups contain sessions, sessions contain subagents/forks — and the lineage panel already established the tree/treeitem convention in this codebase.
• Every row is a treeitem: sessions, services, the operator row, group headers, and the "N archived" rows.
  • Sessions and services carry aria-selected (mirrors the active row).
  • Group headers, archived rows, and sessions with children carry aria-expanded; leaves don't, so they never announce as collapsed empty branches.
  • All rows expose aria-level / aria-posinset / aria-setsize, computed by a small DFS walk over the already-flat orderedItems() output.
  • ARIA attrs are diffed with their own signature in the keyed reconcile, same pattern as class/inner, so unchanged rows aren't touched.
• Real roving tabindex: exactly one row — the caret row if focus is in the list, else the selected row, else the first — holds tabindex="0"; all others -1. Tab from the top of the page reaches the list exactly once, then the existing arrow-key handling takes over. focusListRow promotes the row it focuses and demotes the rest.
• toggleListFocus enters through the roving Tab stop. Its old query looked for .item.selected, a class no row ever carries, so C-x o always landed on the first session row; now it returns to the selected/last-caret row.

Unchanged: all key handling (Enter / Space / Shift+Arrow / group Left/Right, C-n/C-p), rendering, drag, click behavior.

Testing

• New assertion block in web_smoke.rs (web_client_loads_and_websocket_connects) covering: tree/treeitem roles, aria-selected on the selected subagent, aria-expanded on expanded/collapsed groups and a parent session, level/posinset/setsize on nested rows, and the single-Tab-stop invariant.
• No visual change, so no screen recording — the diff is invisible attributes.

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