Add shared Studio toast handling

[thsnkhn]

Summary

• Add a shared Studio toast system with store, host component, and useToast() helper.
• Wire API responses to emit best-effort toast intents alongside existing dialogs without changing request semantics.
• Surface toast feedback in the record form and document the v1 toast contract in docs/toasts.md.

Testing

• Added unit coverage for the API client toast path, toast store behavior, and toast JSON serialization.
• Not run (not requested).

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep toast live region mounted

When the toast list is empty, this v-if removes the aria-live container, so the first toast after an idle period is inserted as part of a newly created live region rather than as a change to an existing one. Screen-reader users can miss initial or single success/error toasts in that scenario; keep the live region mounted and conditionally render only the toast children or hide the empty container instead.

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[thsnkhn]

Fixed in 1b0c02a. The aria-live toast host now remains mounted when the queue is empty, with a regression test.