[LetMeSleep8h]

Fixes #328, exactly the three steps the issue lays out:

1. The composer card (frontend/components/chat/composer.tsx) gains focus-within:border-border-focus-ring focus-within:ring-border-focus-ring focus-within:ring-1 — tabbing to the textarea now draws a visible ring around the card (the textarea itself keeps its outline-none focus-visible:ring-0, since the card is the focus indicator now).
2. The role="listbox" element in slash-command.tsx gets id="slashcmd-listbox", and the textarea's aria-controls points at it instead of the slashcmd-label heading's id — a screen reader now hears that the combobox controls the list, not a paragraph.
3. New render test slash-command-listbox.test.tsx pins the listbox id.

bun test components green (all existing suites + the new one).

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