Browser profiles: share a custom profile's cookies across all environments

[jkracz]

Problem

Browser profiles look global, but their cookies are per environment.

With more than one environment connected, I create a profile and sign in to sites under it (or import cookies from Chrome). Then I open a preview tab under that same profile from a thread in another environment, and I'm signed out of everything. The profile name, the list in Settings → Integrations → Browser profiles, and the settings scope showing "All environments" all suggest one shared identity, but each environment gets its own empty cookie jar behind the same name.

Importing makes it worse. The import wizard always writes into the primary environment's partition, so there's no way in the UI to get imported cookies into the other environments. The workaround is to sign in again in every environment, for every profile.

Evidence from main (3e6b450)

• The profile list is client-local settings, so every environment sees the same profiles (browserProfile.ts).
• The Electron partition is derived from [environmentId, profileId], or just environmentId for Default (resolvePartitionScope).
• Import targets primaryEnvironment.environmentId only (IntegrationsSettings.tsx).

History

• The original preview browser (52c77c1) used one shared partition, with the design note: "The shared persist:t3code-preview partition keeps cookies in sync."
• 6c6740e ("Refine collaborative browser preview") changed it to getBrowserPartition(environmentId). I couldn't find a stated reason for scoping cookies by environment.
• feat(desktop): browser profiles for the preview browser #7254 (browser profiles) kept Default on that legacy partition so upgrading wouldn't sign anyone out, and scoped custom profiles by environment + profile.

Proposal

Make custom profiles one identity across environments: derive their partition from the profile id alone.

• Default stays per environment, so nobody is signed out, which keeps feat(desktop): browser profiles for the preview browser #7254's compatibility guarantee.
• Incognito stays in memory as it is now.
• Import into a custom profile then works for every environment, and clear/remove only has one partition to touch.

Cookies live on the desktop machine either way. Partitions are local to the Electron app and are never sent to a remote environment, so sharing a profile across environments doesn't move any data off the device.

Questions for maintainers

1. Was the environment scoping in 6c6740e intentional? If so, what should it protect against?
2. Existing custom-profile cookies are split across environments. Should the shared partition start empty (users sign in again once), or adopt the primary environment's partition? I lean toward starting empty for simplicity.

If this direction and scope look right, I'm happy to send a PR.