feat: include ongoing timer in balance calculation

The balance now accounts for any active timer whose project belongs to
the tracked client. computeBalance() fetches the user's OngoingTimer,
computes its elapsed seconds, and adds them to the matching period's
tracked seconds before running the balance formula — so both
currentPeriodTrackedSeconds and totalBalanceSeconds reflect the live
timer without requiring a schema change.

On the frontend, useClientTargets polls every 30 s while a timer is
running, and a pulsing green dot is shown next to the balance figure on
the Dashboard and Clients pages to signal the live contribution.

frontend/src/pages/ClientsPage.tsx

@@ -316,6 +316,12 @@ function ClientTargetPanel({
             <span className="font-medium">{target!.targetHours}h</span>/{periodLabel}
           </span>
           <span className={`text-xs font-semibold ${balance.color}`}>{balance.text}</span>
+          {target!.hasOngoingTimer && (
+            <span
+              className="inline-block h-2 w-2 rounded-full bg-green-500 animate-pulse"
+              title="Timer running — balance updates every 30 s"
+            />
+          )}
         </div>
         <div className="flex items-center gap-1">
           <button