fix: pin absolute script path in status-right widget

The #() body runs with the tmux server's PATH; a bare 'party' silently
broke the widget for uninstalled or non-standard-prefix hosts.

tests/90-notifications.bats

@@ -19,12 +19,16 @@ party_rec() { printf '%s/party-%s:%s.d/roster\n' "$PARTY_SOCKET_DIR" "$USER" "$1
   [[ "$out" == *"client-detached"* ]]
 }
 
-@test "status-right widget is configured" {
+@test "status-right widget is configured with an absolute self path" {
   "$PARTY_BIN" host srtest
   rec=$(party_rec srtest)
   sock=$(awk -F= '$1=="SOCKET"{print $2}' "$rec")
   sr=$(tmux -S "$sock" show-options -gv status-right)
   [[ "$sr" == *"party who --short"* ]]
+  # The #() body is executed with the tmux server's PATH, which need
+  # not contain an installed `party` — the widget must pin the absolute
+  # path of the script that hosted the party.
+  [[ "$sr" == "party: #(/"* ]]
 }
 
 @test "client-attached hook invokes a notify helper that broadcasts display-message" {