fix(embed): restore youtube thumbnails and file-row labels (#1148)

* fix(comment-content): render reason text as comment content

* fix(post-form): use native browser styling for flash tag select

Exclude the flash tag dropdown from themed post-form select styling so it
renders with the browser's default select appearance, matching the flag selector.

* fix(p2p-stats): show peer flags for DNS6 relay hostnames

Extract embedded IPv6 addresses from dns6 multiaddrs so geo lookup and country flags work for relay peers that publish IPv6 via DNS hostnames.

* fix(flags): hide geolocation-only selectors on /int/ and /sp/

Country-only boards auto-publish geographic location flags without
showing a flag dropdown, matching existing /bant/ behavior.

* fix(embed): restore youtube thumbnails and file-row labels

Restore thumbnail-first previews for YouTube embeds in post media and markdown hover.
Desktop posts show File with the thumbnail image URL and a youtube video type label.

* fix(embed): address youtube thumbnail review feedback

Translate the youtube video label, handle mobile/music YouTube hosts as standard YouTube URLs, keep affected mocks current, and cap default Vitest workers to reduce local CPU spikes.

public/translations/cs/default.json

@@ -399,5 +399,6 @@
   "ban": "zakázat",
   "timestamp": "časové razítko",
   "tag": "tag",
-  "post_form_flash_upload_prompt": "Recommended SWF host: <catbox>Catbox</catbox>. Upload a .swf, then paste the direct https://files.catbox.moe/...swf link in Link. Other hosts must provide a direct HTTPS .swf URL with CORS enabled."
+  "post_form_flash_upload_prompt": "Recommended SWF host: <catbox>Catbox</catbox>. Upload a .swf, then paste the direct https://files.catbox.moe/...swf link in Link. Other hosts must provide a direct HTTPS .swf URL with CORS enabled.",
+  "youtube_video": "video YouTube"
 }