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>

bin/omarchy-install-chromium-ytdlp

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+# omarchy:summary=Install the native messaging host for the yt-dlp Chromium extension
+
+set -euo pipefail
+
+HOST_NAME="com.omarchy.ytdlp"
+HOST_PATH="$OMARCHY_PATH/bin/omarchy-chromium-ytdlp-host"
+TEMPLATE="$OMARCHY_PATH/default/chromium/native-messaging-hosts/$HOST_NAME.json"
+
+# Chromium-based browser profile roots that use the NativeMessagingHosts layout.
+browser_dirs=(
+  "$HOME/.config/chromium"
+  "$HOME/.config/google-chrome"
+  "$HOME/.config/google-chrome-beta"
+  "$HOME/.config/google-chrome-unstable"
+  "$HOME/.config/BraveSoftware/Brave-Browser"
+  "$HOME/.config/BraveSoftware/Brave-Browser-Beta"
+  "$HOME/.config/BraveSoftware/Brave-Browser-Nightly"
+  "$HOME/.config/microsoft-edge"
+  "$HOME/.config/microsoft-edge-dev"
+)
+
+manifest=$(sed "s|__HOST_PATH__|$HOST_PATH|g" "$TEMPLATE")
+
+for dir in "${browser_dirs[@]}"; do
+  mkdir -p "$dir/NativeMessagingHosts"
+  printf '%s\n' "$manifest" >"$dir/NativeMessagingHosts/$HOST_NAME.json"
+done