fix: indent loose list continuation content under parent bullet

[backnotprop]

Summary

• The markdown parser now merges blank-line-separated indented content into the preceding list item as a "loose continuation," matching standard markdown behavior
• The renderer splits multi-paragraph list items on \n\n and renders each segment as a separate <p> under the bullet, keeping content visually indented
• Same fix applied to the plan diff view (both unchanged and modified block renderers)
• Tight continuation (no blank line) preserves the original \s+ indent check; only loose continuation (after blank line) requires \s{2,}

Closes #704

Test plan

• Run ./tests/manual/local/test-loose-list.sh and walk through all 16 sections of the fixture — each section describes expected rendering and what a regression looks like
• Run bun test packages/ui/utils/parser.test.ts — all 103 tests pass
• Test plan diff view with a plan containing loose list items (deny, resubmit with changes to a loose item body)