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-capture-screenrecording

@@ -228,7 +228,7 @@ stop_screenrecording() {
 
     omarchy-notification-send "Screen recording saved" "Open with Super + Alt + , (or click this)" \
       -t 10000 --image "${preview:-$filename}" \
-      --exec "$(printf 'mpv %q' "$filename")"
+      --exec-arg mpv --exec-arg -- --exec-arg "$filename"
 
     # 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