Fix the polkit unit's ignored rate limit

systemd logged "Unknown key 'StartLimitIntervalSec'" on every start:
rate limiting belongs in [Unit], not [Service], so both keys were
ignored and the restart limit they were meant to impose did not exist.
An agent that failed repeatedly would have flapped rather than stopping.

Found while investigating reported prompt failures, which turned out not
to be a defect: the trace showed both authentications succeeding on the
first attempt. The failures came from testing -- prompts raised by
background pkexec runs and then cancelled seconds later, while someone
was trying to type into them.

Claude-Session: https://claude.ai/code/session_01BRvzt4H8XXLPVH5MyYdk9L

config/dot/quickshell/services/Polkit.qml

@@ -43,6 +43,7 @@ Singleton {
     // Held only between pressing Enter and the helper accepting it on stdin.
     property string pendingSecret: ""
 
+
     function begin(path: string): void {
         // A second request while one is open would leave the first
         // unanswerable; polkit serializes these in practice, and refusing is
@@ -84,6 +85,8 @@ Singleton {
         helper.running = true;
     }
 
+    // Cancelled by the person at the keyboard, or withdrawn by whatever asked.
+    // Both close the prompt; only the first is something they did.
     function cancel(): void {
         root.dismiss("cancelled");
     }