/iblai-vibe-billing

Add the tenant Billing surface -- "Manage your billing and subscription" for the whole workspace, in three tabs: Plan & Credits (current plan with Stripe upgrade, credit balance with its USD equivalent, Add Credits, and Auto Recharge with threshold / recharge amount / spending limit), Spend Limits (the single workspace-wide LLM spend limit covering everything all agents and users spend), and Agent Limits (every agent spend cap in the workspace in one searchable table — each row opens the same per-agent Billing editor documented in /iblai-vibe-agent-billing).

This is the tenant-level counterpart of the per-agent Billing tab: per-agent and per-user limits live in each agent's settings (/iblai-vibe-agent-billing); this surface holds the plan/credits management, the workspace-wide cap, and the cross-agent overview.