fix: show "Unlimited budget" when budget is $0

When budgetCents is 0 (no budget set), the UI now shows "Unlimited
budget" instead of misleading "0% of $0.00 budget" throughout:

- Dashboard: metric card shows "Unlimited budget" instead of percentage
- Companies: shows "Unlimited budget" label instead of "/ $0.00"
- Costs: hides utilization percent and progress bar, shows "Unlimited budget"
- Inbox: suppresses budget utilization alert when no budget is set

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/pages/Dashboard.tsx

@@ -211,7 +211,9 @@ export function Dashboard() {
               onClick={() => navigate("/costs")}
               description={
                 <span className="cursor-pointer" onClick={() => navigate("/costs")}>
-                  {data.costs.monthUtilizationPercent}% of {formatCents(data.costs.monthBudgetCents)} budget
+                  {data.costs.monthBudgetCents > 0
+                    ? `${data.costs.monthUtilizationPercent}% of ${formatCents(data.costs.monthBudgetCents)} budget`
+                    : "Unlimited budget"}
                 </span>
               }
             />