fix(embed): video sizing letterbox and logo accent

- Changed embed video sizing to `object-fit: cover` and `width: 100%` to eliminate black bars/letterboxing.
- Updated `embed.html` and `css/tailwind.source.css` to enforce these styles on `.video-modal__media`, `.video-modal__video`, and `#embedVideo`.
- Wired the embed logo accent color to use the instance configuration (`THEME_ACCENT_OVERRIDES` from `config/instance-config.js`) via a `--bitvid-accent` CSS variable injected in `js/embed.js`.
- Added unit and visual tests to verify styling and configuration logic.

embed.html

@@ -79,7 +79,7 @@
       >
         <video
           id="embedVideo"
-          class="h-full w-full object-contain bg-transparent"
+          class="h-full w-full object-cover bg-transparent"
           controls
           playsinline
           preload="metadata"