fix(billing): redirect free workspace to checkout instead of alerting

[ABB65]

Summary

• A free workspace clicking Connect repository hit POST /api/workspaces/:id/projects and the server correctly returned 402 { requiresCheckout: true } — but ConnectRepoDialog only surfaced a toast.error, never opened the plan-selection modal. User saw an alert and got stuck.
• Same shape applied to every other workspace-scoped composable: useChat, useBranches, useContentEditor, useMembers, ContentCollectionView, etc. all funnel API errors through resolveApiError → toast.error. A billing-locked workspace (trial_expired / grace_expired / canceled_expired) emitted by server/middleware/03.billing.ts would silently toast across the app.
• Fix centralizes the redirect in one place:
  • New usePlanModal composable — useState-backed global open state.
  • resolveApiError opens the modal as a side effect when status is 402 + requiresCheckout. All existing catch sites now route locked workspaces through checkout for free.
  • default layout mounts OrganismsPlanSelectionModal (previously only mounted in workspace layout / settings panels / error.vue).
  • workspace layout swaps its local planModalOpen ref for the shared composable so TrialBanner + middleware 402s drive the same modal.
  • ConnectRepoDialog closes its own dialog on 402 + requiresCheckout so the plan modal lands cleanly instead of stacking.

Why this is the right shape

resolveApiError is already the single client-side entry point for API errors. Adding the side effect there (instead of patching every composable or introducing a custom $fetch instance) keeps the change small and ensures any new call site that follows the existing pattern is covered without extra wiring.

Test plan

• Sign up → land on free workspace → click Connect repository → after picking a repo, Connect opens PlanSelectionModal (not a toast). The repo dialog is closed.
• Choose Starter — redirected to Polar checkout.
• Subscribed/trial workspace: connect flow still works end-to-end (no regression).
• Trial-expired workspace: any workspace-scoped action (send message, save content, invite member, merge branch) opens the plan modal.
• Community / on-premise profiles: hasManagedBilling is false → modal is not mounted; UI degrades to existing toast behavior.