web UI: session list has keyboard nav but no listbox/option accessibility semantics #1075

The web UI session list has good keyboard navigation but no accessibility semantics, so assistive tech announces it as a stack of anonymous divs.

Current state (working, keep it)

handleSessionListKey (crates/daemon/assets/index.html ~line 4907 @ 4c1459b) already implements TUI-parity keyboard control: Up/Down and C-n/C-p to move, Enter to drill into the view, Space to pin (or collapse a group), Left/Right to collapse/expand groups, Shift+Arrow to reorder. focusListRow() does roving focus with tabindex="-1". None of that needs changing.

The gap

• Rows are <div class="item"> with no role. The container has no role="listbox". A screen reader gets no list structure, no item count, no position.
• No aria-selected on the active row — selection is communicated by a CSS class only.
• No tabindex="0" entry point: the list is reachable only via the focus-toggle chord, so a Tab-only user never reaches it. focusListRow sets tabindex="-1" at focus time, which supports roving focus but never puts the list in the tab order.
• Group headers have the same problem and additionally need expanded/collapsed state exposed.

Expected

• Container gets role="listbox" (or tree, given group nesting and depth classes), rows get role="option" with aria-selected, and group headers get aria-expanded.
• The currently selected row carries tabindex="0" and all others -1, so Tab reaches the list once and the arrow keys take over from there — the standard roving-tabindex pattern the code is already 90% of the way to.
• Depth is exposed via aria-level / aria-posinset / aria-setsize rather than being inferable only from left padding.

Acceptance criteria

• A screen reader announces the list as a list, each row with its name and selected state, and group headers with expanded/collapsed state.
• Tab from the top of the page reaches the session list exactly once; arrow keys then navigate within it.
• Existing key handling (Enter / Space / Shift+Arrow / group Left-Right) is unchanged.

Found during a power-user UX audit of the TUI and web UI.