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 {

apps/client/src/features/ai-chat/components/reasoning-block.tsx

@@ -3,6 +3,7 @@ import { Box, Collapse, Group, Text, UnstyledButton } from "@mantine/core";
 import { IconChevronDown } from "@tabler/icons-react";
 import { useTranslation } from "react-i18next";
 import { estimateTokens } from "@/features/ai-chat/utils/count-stream-tokens.ts";
+import { collapseBlankLines } from "@/features/ai-chat/utils/collapse-blank-lines.ts";
 import { renderChatMarkdown } from "@/features/ai-chat/utils/markdown.ts";
 import classes from "@/features/ai-chat/components/ai-chat.module.css";
 
@@ -33,7 +34,12 @@ export default function ReasoningBlock({ text, tokens }: ReasoningBlockProps) {
   // Authoritative count wins; otherwise estimate live from the streamed text.
   const count = tokens && tokens > 0 ? tokens : estimateTokens(text);
   const trimmed = text.trim();
-  const html = trimmed ? renderChatMarkdown(trimmed, {}) : "";
+  // Collapse the blank-line gaps the model emits between every list item /
+  // paragraph so the reasoning renders compactly (tight lists, joined
+  // paragraphs) — see collapseBlankLines. ONLY here, not in the normal answer.
+  const html = trimmed
+    ? renderChatMarkdown(collapseBlankLines(trimmed), {})
+    : "";
 
   return (
     <Box className={classes.reasoningBlock} mb={6}>