Render assistant chat responses as rich text

[jerelvelarde]

Summary

• Render assistant chat responses as rich text across web, iOS, and Android while leaving user messages literal.
• Preserve selection for prose and code blocks, restrict opened links to HTTP(S), and show image alt text without fetching remote images.

Verification

• 159 tests, typecheck, lint, and server/web/iOS/Android builds pass.
• Browser component preview verified formatting, line breaks, link opening, and text selection.
• Native device UI was not exercised.