Add first-run onboarding for new users #263

#261 removed the welcome screen and the driver.js tour. We still want onboarding, but built differently.

Why the old tour went

• It was a "what's new" walkthrough for users coming from the legacy site (sidebar, shocker layout options, the user-shares tabs, OAuth connections). It barely touched the path a new user actually has to take.
• It found its targets through data-tour attributes and sidebar selectors across four pages, and moved between pages with goto. When markup moved, a step would point at nothing, and nothing caught it.
• After the welcome screen's sunset it could no longer be reached.

What we want

Guide a new account through the real first-run path:

1. Add a hub
2. Pair a shocker to it
3. Control it (and optionally share it)

Suggested approach

Prefer contextual empty states over a scripted tour, for example:

• Shockers page with no hubs: explain that a hub comes first and link to adding one.
• Hubs page with no hubs: what a hub is, how to flash and pair one, link to the docs/flashtool.
• Shares pages with nothing shared yet: a short line on what shares are for.

These sit where the user already is, need no extra state, and can't drift out of sync with the pages.

Notes

• The old implementation is in history before chore: remove welcome screen and onboarding tour #261 (src/lib/tour/, src/routes/WelcomeScreen.svelte) if any copy is worth reusing.
• chore: remove welcome screen and onboarding tour #261 also clears the old os.welcomeVersion / os.tourCompletedVersion localStorage keys, so any new onboarding state should use fresh keys.