feat(app): show ChatGPT usage limit dialog - #52153

Summary

• Show the ChatGPT usage-limit dialog in desktop and web when a failed V2 step has the backend-mapped message for subscription_sharing_usage_limit_exceeded.
• Offer ChatGPT usage settings and a Close button, with a 24-hour repeat cooldown. There is no provider-switching action.
• Keep the change scoped to packages/app; no backend or generated-client changes.

Validation

• bun run check and focused detector tests passed; the final platform-gating removal passed bun run check.
• Production timeline streaming benchmark passed before and after the platform-gating change (single samples; not a statistically significant performance claim).

Follow-up

• The backend token-sharing port must map this exact message and stop retries for the usage-limit error. Matching the message is intentionally simple but will need updating if the backend copy changes. The frontend does not change retry or billing behavior.