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/Companies.tsx

@@ -247,12 +247,11 @@ export function Companies() {
                 <div className="flex items-center gap-1.5">
                   <DollarSign className="h-3.5 w-3.5" />
                   <span>
-                    {formatCents(company.spentMonthlyCents)} /{" "}
-                    {formatCents(company.budgetMonthlyCents)}
+                    {formatCents(company.spentMonthlyCents)}
+                    {company.budgetMonthlyCents > 0
+                      ? <> / {formatCents(company.budgetMonthlyCents)} <span className="text-xs">({budgetPct}%)</span></>
+                      : <span className="text-xs ml-1">Unlimited budget</span>}
                   </span>
-                  {company.budgetMonthlyCents > 0 && (
-                    <span className="text-xs">({budgetPct}%)</span>
-                  )}
                 </div>
                 <div className="flex items-center gap-1.5 ml-auto">
                   <Calendar className="h-3.5 w-3.5" />