Run notification click actions as argv, not shell strings

The click action of a notification was a free-form shell string run through
`bash -lc`, safe only when every sender shell-quoted every interpolated value
perfectly. One slip is RCE: a hostile yt-dlp video title forged an output
record and injected an mpv option into the click command (mehmetince.net RCE,
partially addressed by #7847).

Add a parameterized transport: omarchy-notification-send gains --exec-arg
(repeatable), encoding a JSON argv into the omarchy-exec-argv hint. The shell
runs it with Quickshell.execDetached(argv) and no shell, so data an attacker
controls is only ever one argument and can never be reparsed as a command. The
shell fails closed on a malformed argv hint.

The legacy free-form --exec string is retained but honored only from Omarchy's
own omarchy-action toasts, and deprecated. Migrate all in-repo callers
(screenshot, screen recording, taildrop receive, migrate-notify, crash-watch,
yt-dlp host) to --exec-arg. Update docs and tests.

bin/omarchy-chromium-ytdlp-host

@@ -70,10 +70,10 @@ title_from_file() {
   fi
 }
 
-# `--` keeps a path that starts with `-` from being parsed as an mpv option.
-playback_command() {
-  printf 'mpv -- %q' "$1"
-}
+# The click action is passed to the shell as an argv vector (--exec-arg), so the
+# path is one literal argument and never reaches a shell. `--` still guards mpv
+# itself against parsing a leading-dash filename as an option.
+playback_exec_args=(--exec-arg mpv --exec-arg -- --exec-arg)
 
 # Drive the Quickshell OSD — a single overlay that updates in place (like the
 # volume/brightness bar), so download progress never stacks like notifications.
@@ -153,7 +153,7 @@ download_url() {
     # toast would exit before the thumbnail cleanup below is ever scheduled.
     omarchy-notification-send -g 󰄬 "Download complete" "$title" \
       -t 10000 --image "${preview:-$filepath}" \
-      --exec "$(playback_command "$filepath")" || true
+      "${playback_exec_args[@]}" "$filepath" || true
 
     # The shell loads the thumbnail into memory when the toast appears and never
     # re-reads the file, so the preview only has to outlive that load, not the