Fix missing display: flex on caddl thumbnail inner div (#1728)

## Summary

Fixes a CSS issue in the CAD/3D asset thumbnail template where flex
properties were being applied without `display: flex`.

## Problem

The inner div in `archivebox/plugins/caddl/templates/thumbnail.html` had
`flex-direction: column` and `align-items: center` styles, but was
missing `display: flex`. Without the display property set to flex, these
flex properties have no effect, so the emoji (🧊) and "3D Asset" text
weren't stacking vertically as intended.

## Solution

Added `display: flex;` to the inner div's style attribute so the flexbox
layout takes effect and the content stacks correctly.

## Changes

- `archivebox/plugins/caddl/templates/thumbnail.html`: Added `display:
flex;` to line 3

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixed the CAD/3D asset thumbnail layout by adding display: flex to the
inner div so flexbox applies and the 🧊 emoji and "3D Asset" label stack
vertically and stay centered.

<sup>Written for commit a01bdd409a.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

archivebox/plugins/caddl/templates/thumbnail.html

@@ -1,6 +1,6 @@
 <!-- CAD/3D asset thumbnail -->
 <div class="extractor-thumbnail caddl-thumbnail" style="width: 100%; height: 100px; overflow: hidden; background: #1a1a1a; display: flex; align-items: center; justify-content: center;">
-    <div style="flex-direction: column; align-items: center; color: #888; font-size: 12px;">
+    <div style="display: flex; flex-direction: column; align-items: center; color: #888; font-size: 12px;">
         <span style="font-size: 32px;">🧊</span>
         <span>3D Asset</span>
     </div>