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-screenshot

@@ -70,7 +70,7 @@ case "$PROCESSING" in
     # notification outage must not report the capture itself as failed.
     omarchy-notification-send "Screenshot saved to clipboard and file" "Edit with Super + Alt + , (or click this)" \
       --image "$FILEPATH" \
-      --exec "$(printf '%q %q' "$SCREENSHOT_EDITOR" "$FILEPATH")" || true
+      --exec-arg "$SCREENSHOT_EDITOR" --exec-arg "$FILEPATH" || true
     ;;
   copy)
     grim -g "$SELECTION" - | wl-copy --type image/png