feat(desktop): add first-run onboarding tab

[adamleithp]

Problem

New PostHog Desktop users have no persistent place to learn the main concepts after setup.

The setup flow ends on an active session, so onboarding must not replace that work.

Changes

• First runs now add an inactive Onboarding tab after the active session.
• The page explains Spaces, Multiplayer, Self-driving, Canvases, Agents, and Tasks.
• Each concept link opens a new app tab, so the onboarding page stays open.
• The Onboarding tab uses a pulsing sparkle icon and stops the motion when reduced motion is active.
• The page hides Self-driving when the destination is unavailable.
• The app records onboarding page views and destination selections.
• The app uses the existing provisioning signal and stores one insertion marker per project identity.

Before: PostHog Desktop had no onboarding tab or landing page.

After:

How did you test this code?

• Scoped Vitest tests protect background activation, one-time insertion, destination mappings, and availability gating.
• pnpm typecheck checked the complete Desktop workspace.
• The host boundary check passed.
• The Storybook build passed and produced the screenshot above.
• hogli ci:preflight --fix ran Desktop Biome on every changed file.
• The task shell could not run pnpm lint because it invoked ESLint without a configuration file.
• The Electron first-account flow was not tested manually.

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

Automatic notifications

• Publish to changelog?

Docs update

The browser tabs guide now documents the first-run tab and inactive tab creation.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Pi used Explore subagents and Playwright.
• The task applied /posthog-desktop, /writing-ui-components, /writing-user-facing-copy, /writing-tests, and /writing-code-comments.
• The task also applied /instrument-product-analytics, /running-ci-preflight, /writing-pr-descriptions, /subagent-orchestration, and /writing-simplified-technical-english.
• An open pull request search found no matching implementation.

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Created with PostHog Desktop

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit f5e5bed.}

[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

Latest build (f5e5bed): https://93e875b4612747049f30192785658a67.hosthog.dev

Employee-gated; every push gets a fresh URL whose content never changes. All previews stop serving when the PR closes.