/iblai-vibe-agent-billing

Add the agent Billing tab -- set LLM spend limits for this agent and its users, and see how much has been used. Two sub-tabs match the backend's agent-level scopes: This Agent (the agent's own spend cap) and Per User (explicit per-user limits on this agent, with the user picked through a platform-user search). Each limit has an enable toggle, a usage bar (spent / % used / remaining), a spend amount with a reset interval (day/week/month/year), an enforcement mode (Block Requests or Alert Only), and comma-separated alert thresholds at which admins get a near-limit alert. Enforcement is server-side — a blocked request is refused with a 429 / spend_cap_exceeded error until the period resets. This is one tab in the wider agent-settings family. All tabs share the same AgentSettingsProvider wrapper.