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-server/src/engine/core-modules/record-crud/zod-schemas/record-properties.zod-schema.ts

@@ -256,8 +256,14 @@ export const generateRecordPropertiesZodSchema = (
     }
 
     if (field.name === 'position') {
+      fieldSchema = z.union([
+        z.number(),
+        z.literal('first'),
+        z.literal('last'),
+      ]);
+
       fieldSchema = fieldSchema.describe(
-        'Leave empty to place at the top of the list (recommended).',
+        'Use "first" to insert at the top, "last" for the bottom, or a number for explicit ordering. Leave empty to place at the top (recommended).',
       );
     } else if (field.description) {
       fieldSchema = fieldSchema.describe(field.description);