feat(desktop): add Dev Cloud region

[dmarticus]

Problem

Desktop developers cannot sign in to the shared Dev Cloud deployment without changing the local-only development region.

Why: Deployed OAuth integrations (such as Slack) need a hosted test path while existing Local development sessions keep using localhost.

Changes

• Development builds show Dev Cloud above Local development. Production builds still show only US and EU.
• The new region uses a dedicated OAuth client and routes API and agent traffic through the Dev Cloud hosts.
• Region schemas, stored authentication, IPC, and agent RPC contracts accept dev-cloud. The existing dev value stays unchanged.

How did you test this code?

• The development app showed both development choices and started Dev Cloud OAuth on the existing localhost callback port.
• Relevant automated tests cover mappings, build visibility, serialization, stored-session restoration, and agent routing. Full Desktop lint and typecheck passed.
• Authenticated project, MCP server, and agent checks remain untested because this VM has no Dev Cloud browser session.

👉 Stay up-to-date with PostHog coding conventions for a smoother review.

Automatic notifications

• Publish to changelog?

Docs update

Updated the local-development guide to explain Local development, Dev Cloud, and the separate analytics host setting.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Codex used the Desktop development build, Storybook, and GitHub tooling. Invoked skills: posthog-desktop, writing-tests, writing-user-facing-copy, test-electron-app, and writing-pr-descriptions.

The final design adds a separate hosted region and keeps the existing local region compatible.

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Created with PostHog Desktop

[trunk-io]

😎 Merged successfully - details.

[github-actions]

🤖 CI report

✅ Trunk lane — non-backend lane

This PR is assigned to the non-backend lane. It does not run backend Python tests and may merge in parallel with PRs in other lanes.

[hosthog]

HostHog preview — posthog-desktop-web

The previews for this PR have been torn down and no longer serve.

[github-actions]

React Doctor found 1 issue in 1 file · 1 warning.

1 warning

packages/ui/src/features/auth/RegionSelect.tsx

• ⚠️ packages/ui/src/features/auth/RegionSelect.tsx:23 Non-component export in component file only-export-components

_{Reviewed by React Doctor for commit 173159b.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(desktop): list Dev Cloud before loca..." | Re-trigger Greptile}

[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-09-04 19:19 UTC	Run
prod-us	✅ Deployed	2026-09-04 19:36 UTC	Run
prod-eu	✅ Deployed	2026-09-04 19:30 UTC	Run