fix(markdown): add imgui.spacing() after each markdown chunk render

User reported that nested list items in the Discussion Hub's read_mode
entries were overlapping with adjacent text (e.g., '- gte_lw(...)'
overlapping with 'These are different things...' below it). This is
the imgui_md library's known issue with list item line height.

FIX: Add an imgui.spacing() call after each imgui_md.render() to force
a small vertical gap between chunks. This prevents adjacent list items
from rendering at overlapping Y positions.

Tests: 16/16 broad regression pass

src/markdown_helper.py

@@ -135,8 +135,8 @@ class MarkdownRenderer:
     chunk = "".join(md_buf)
     if chunk.strip():
      imgui_md.render(chunk)
+     imgui.spacing()
     md_buf.clear()
-
   def flush_code() -> None:
    nonlocal block_idx
    if code_buf: