fix(a11y): add keyboard support to interactive non-button elements

[akinshaywai]

Problem

Two interactive elements were only accessible via mouse, failing WCAG 2.1 SC 2.1.1 (Keyboard):

1. ReflectionItem in cards.tsx

A <span> used as a toggle to expand/collapse reflection text. It had an onClick handler but:

• No role="button" — screen readers didn't identify it as interactive
• No tabIndex — keyboard users couldn't focus it
• No onKeyDown — pressing Enter or Space did nothing

2. Session list item in HistoryList.tsx

A <div role="button" tabIndex={0}> was already focusable, but had no onKeyDown handler, so keyboard users who pressed Enter or Space to activate it got no response.

Fix

cards.tsx — ReflectionItem:

• Added role="button", tabIndex={0}, aria-expanded={expanded}
• Added onKeyDown to toggle on Enter / Space

HistoryList.tsx:

• Added onKeyDown to match the existing onClick behaviour

Test Plan

• Focus the reflection text items with Tab and press Enter/Space — they should expand/collapse
• Focus a session history row with Tab and press Enter/Space — it should open the session