Fix margin issue in markup paragraph rendering (#34599) (#34606)

Backport #34599 by @silverwind

The Fomantic-inherited `p:last-child` rule in base.css interferes with
this markdown rendering:

```
1. a

2. a
```

Before (unequal margin):
<img width="143" alt="Screenshot 2025-06-04 at 14 09 07"
src="https://github.com/user-attachments/assets/6d6ba5e0-0d84-42e0-a0e4-9e93a59c4d65"
/>

After (rendering matches GitHub):
<img width="181" alt="Screenshot 2025-06-04 at 14 09 14"
src="https://github.com/user-attachments/assets/bb81e14e-bc9f-4d52-92d0-f7a17c63070e"
/>

Co-authored-by: silverwind <[email protected]>

Author: GiteaBot

web_src/css/markup/content.css

@@ -134,6 +134,11 @@
   margin-bottom: 16px;
 }
 
+/* override p:last-child from base.css */
+.markup p:last-child {
+  margin-bottom: 16px;
+}
+
 .markup hr {
   height: 4px;
   padding: 0;