fix(ui): keep partial markdown readable while responses stream

[nexxeln]

Summary

• keep partially streamed markdown readable instead of showing broken inline formatting while an answer is still arriving
• stop unfinished links from turning into weird half-broken clickable markup before the model finishes the URL
• preserve the open-code-fence behavior from the previous fix while adding tests for the streaming cases that were easiest for users to notice

Testing

• bun test "src/components/markdown-stream.test.ts" (in packages/ui)
• bun typecheck (in packages/ui)
• bun typecheck (in packages/app)