feat: Resolve best seat across orgs for rate limits

[charlesvien]

Problem

Users with a pro plan in one org were getting free-tier rate limits in other orgs because the gateway resolved plans per (user_id, team_id) instead of the user's best plan across all orgs.

Changes

1. Pass best=true to getMySeat so the API returns the highest-tier seat across all orgs
2. Add organization_id and organization_name to SeatData type
3. Track billingOrgId in seat store to switch org context before opening billing
4. Switch to the billing org via switchOrganization before opening the billing page
5. Use server-provided redirectUrl for billing errors instead of hardcoding

How did you test this?

Manually

[charlesvien]

• feat: Resolve best seat across orgs for rate limits #1884 👈 (View in Graphite)
• main

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[greptile-apps]

Prompt To Fix All With AI

This is a comment left during a code review.
Path: apps/code/src/shared/types/seat.ts
Line: 20

Comment:
**Unused field added to type**

`organization_name` is added to `SeatData` but is never read in any of the changed files (only `organization_id` is consumed). This violates the simplicity principle of having no superfluous parts. If it's needed for a future feature, it should be added in that PR.

How can I resolve this? If you propose a fix, please make it concise.

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