Add yt-dlp "Download Video" Chromium extension

Download the current page's video with yt-dlp via Alt+Shift+D or a click
on the toolbar icon. A native-messaging host runs the download, shows live
progress on the Quickshell OSD, and posts a clickable "Download complete"
toast that opens the file in mpv.

- Extension: pinned key for a stable id, green download-video icon,
  keyboard command + toolbar action (reads the active tab URL).
- Native host (omarchy-chromium-ytdlp-host): verifies the URL with
  yt-dlp --simulate (else "No video found"), streams progress to the OSD
  (time-throttled to ~4/s), saves to ~/Videos, opens mpv on click.
- Installer (omarchy-install-chromium-ytdlp) writes the native-messaging
  manifest into installed Chromium/Chrome/Brave/Edge profiles; wired into
  browser install and chromium refresh, with a migration for existing users.
- omarchy-osd: add -d/--duration so the OSD can persist during a download.
- Add yt-dlp to base packages, load the extension via --load-extension,
  and document the Alt+Shift+D binding.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

config/chromium-flags.conf

@@ -1,4 +1,4 @@
 --ozone-platform=wayland
 --ozone-platform-hint=wayland
 --enable-features=TouchpadOverscrollHistoryNavigation
---load-extension=~/.local/share/omarchy/default/chromium/extensions/copy-url
+--load-extension=~/.local/share/omarchy/default/chromium/extensions/copy-url,~/.local/share/omarchy/default/chromium/extensions/yt-dlp