fix(a11y): add screen-reader landmarks and live region for streaming responses

[felladrin]

Root Cause

Issue #2186: The streaming AI answer body had no aria-live region, so assistive-tech users heard nothing as tokens arrived. Separately, the main page lacked a <main> landmark and <h1>, giving no document outline or skip target for screen-reader navigation.

What Changed

File	Change
client/components/Pages/Main/MainPage.tsx	Wrap content in <main> landmark; add visually-hidden <h1> with value proposition
client/components/AiResponse/AiResponseContent.tsx	Pass aria-live="polite" and aria-busy to FormattedMarkdown
client/components/AiResponse/FormattedMarkdown.tsx	Accept and forward aria-live / aria-busy props to the Typography wrapper
client/components/AiResponse/FormattedMarkdown.test.tsx	New — tests for aria attribute forwarding
client/components/Pages/Main/MainPage.test.tsx	Added tests for <main> landmark and hidden <h1>

How Verified

• All 275 existing tests pass (full suite)
• 2 new tests for landmarks on MainPage
• 2 new tests for aria attribute forwarding on FormattedMarkdown
• knip ran clean (auto-fixed 2 unused imports)

[felladrin]

Closing in favor of #2211, which takes the same fix with a more robust live-region placement (a stable always-mounted wrapper rather than threading aria props through the reusable FormattedMarkdown). The test coverage from this PR has been ported onto #2211.