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-tailscale-receive

@@ -72,7 +72,7 @@ announce() {
 
   # Announcing is best-effort: the file is already delivered, and under `set -e`
   # a notification outage would otherwise kill the long-running receiver.
-  omarchy-notification-send "${args[@]}" --exec "$(printf 'xdg-open %q' "$path")" || true
+  omarchy-notification-send "${args[@]}" --exec-arg xdg-open --exec-arg "$path" || true
 }
 
 deliver() {