Improve AI chat UX (#17974)

## Summary
- update AI chat message typography and list line-height for readability
- apply richer markdown-section styling for headings, spacing,
separators, and inline code
- keep links non-underlined by default with underline on hover, using
accent11 for link color
- preserve previous AI chat table design while keeping other markdown
improvements

## Validation
- yarn eslint
packages/twenty-front/src/modules/ai/components/LazyMarkdownRenderer.tsx
packages/twenty-front/src/modules/ai/components/AIChatMessage.tsx

---------

Co-authored-by: Félix Malfait <felix.malfait@gmail.com>

packages/twenty-front/src/modules/ai/utils/groupContiguousThinkingStepParts.ts

@@ -0,0 +1,44 @@
+import { type ExtendedUIMessagePart } from 'twenty-shared/ai';
+
+import { type AssistantMessageRenderItem } from '@/ai/utils/assistantMessageRenderItem';
+import { isThinkingStepPart } from '@/ai/utils/isThinkingStepPart';
+import { type ThinkingStepPart } from '@/ai/utils/thinkingStepPart';
+
+export const groupContiguousThinkingStepParts = (
+  parts: ExtendedUIMessagePart[],
+): AssistantMessageRenderItem[] => {
+  const renderItems: AssistantMessageRenderItem[] = [];
+  let currentThinkingParts: ThinkingStepPart[] = [];
+
+  const flushThinkingParts = () => {
+    if (currentThinkingParts.length > 0) {
+      renderItems.push({
+        type: 'thinking-steps',
+        parts: currentThinkingParts,
+      });
+      currentThinkingParts = [];
+    }
+  };
+
+  for (const part of parts) {
+    if (part.type === 'step-start') {
+      continue;
+    }
+
+    if (isThinkingStepPart(part)) {
+      currentThinkingParts.push(part);
+      continue;
+    }
+
+    flushThinkingParts();
+
+    renderItems.push({
+      type: 'part',
+      part,
+    });
+  }
+
+  flushThinkingParts();
+
+  return renderItems;
+};