feat(ai-chat): compact reasoning rendering — collapse blank lines (#181)

The "Thinking" (reasoning) block rendered with large vertical gaps: models
emit reasoning with a blank line (\n\n) between every list item and
paragraph, which `marked` turns into loose lists (each <li> wrapped in a
<p>) and separate <p> paragraphs, each carrying a margin.

- Add `collapseBlankLines(text)`: collapse 2+ newlines to one, EXCEPT inside
  fenced code blocks (``` / ~~~) where blank lines are significant. Applied
  in reasoning-block.tsx before renderChatMarkdown, so loose lists become
  tight (no <li><p>) and paragraphs join; `breaks: true` keeps single \n as
  <br>, preserving line breaks. Reasoning-only — the normal answer is
  untouched.
- Drop `white-space: pre-wrap` from `.reasoningText`: on the rendered
  markdown <div> it turned the newlines between block tags into visible
  blank lines on top of the margins. The plain-text fallback <Text> that
  needs pre-wrap already sets it inline.

Tests: collapseBlankLines unit (collapse, fence preservation incl. tilde and
unclosed fences) + rendered-HTML assertions that a blank-line-separated list
becomes a tight list and still parses as a list after a paragraph.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/ai-chat/components/ai-chat.module.css

@@ -161,7 +161,11 @@
     margin-top: 4px;
     font-size: var(--mantine-font-size-xs);
     color: light-dark(var(--mantine-color-gray-7), var(--mantine-color-dark-1));
-    white-space: pre-wrap;
+    /* NOTE: `white-space: pre-wrap` is intentionally NOT set here. On the
+       rendered markdown <div> it would turn the newlines between block tags
+       (</li>\n<li>, </p>\n<ol>) into visible blank lines/indents on top of the
+       margins. The plain-text fallback <Text> that needs pre-wrap sets it
+       inline itself (see reasoning-block.tsx). */
 }
 
 .reasoningText p {