Apply margin to all pre in .nicegui-markdown (zauberzeug#5266)

### Motivation

Fix zauberzeug#5264, where the margin is inconsistent when `language=''`

### Implementation

* Changed the selector from `.nicegui-markdown .codehilite pre` to
`.nicegui-markdown pre` and updated the margin to `0.5rem 0.5rem` to
provide both horizontal and vertical margin to both codehilite and
non-codehilite code blocks.
* Removed unnecessary padding from `.nicegui-code .codehilite`. 

### Visual difference

Before:

<img width="600" alt="image"
src="https://github.com/user-attachments/assets/48ce0232-bb6d-4d03-be48-a2a15e6d84c0"
/>

After: 

<img width="600" alt="image"
src="https://github.com/user-attachments/assets/2e306fdb-12c5-49a0-8978-019f9e7fc059"
/>


### Progress

- [x] I chose a meaningful title that completes the sentence: "If
applied, this PR will..."
- [x] The implementation is complete.
- [x] Pytests have been added (or are not necessary).
- [x] Documentation has been added (or is not necessary).

---------

Co-authored-by: Falko Schindler <[email protected]>

Author: evnchn

nicegui/static/nicegui.css

@@ -195,7 +195,7 @@
   padding: 0.5rem;
   border: 1px solid #8884;
 }
-.nicegui-markdown .codehilite pre {
+.nicegui-markdown pre {
   margin: 0.5rem 0;
 }
 .nicegui-markdown .mermaid-pre > .mermaid {
@@ -257,7 +257,7 @@ h6.q-timeline__title {
   box-shadow: 0 0 0.5em rgba(127, 159, 191, 0.05);
   border-radius: 0.25rem;
 }
-.nicegui-code .codehilite {
+.nicegui-code pre {
   padding: 0 0.5rem;
 }
 .nicegui-matplotlib > svg,