feat(billing): usage bar component + enhanced upgrade prompt #3740

Context

Projects need to show usage progress (e.g. "42/100 executions this month") and contextual upgrade nudges.

Spec

• <BillingUsageBar resource="scraps" action="execute" /> — progress bar with label
• Auto-colors: green (<70%), orange (70-90%), red (>90%)
• <BillingUpgradePrompt> enhanced: accepts resource + action props, shows specific limit info
• Both use useQuota() composable internally

Acceptance

• Components + unit tests
• Works with any resource/action
• Depends on: useQuota composable