Svelte (and Vue) bindings can't reconnect with a new auth token without a page reload

Problem

The common auth flow for a SpacetimeDB web app is "start anonymous, then the user signs in" — you open an anonymous connection, the user authenticates (e.g. via Google), you get a logged-in token, and you want to reconnect with it. In the Svelte binding there's no clean way to do this:

• The provider holds its connection in a module-level singleton that's reused across remounts, so even a {#key} swap keeps the old token.
• There's no reconnect path on the context value.
• A DbConnection's token is immutable after build(), so you can't mutate your way out either.

The only thing that actually works today is a hard window.location.reload(), which throws away all client state and flickers the UI on every sign-in/sign-out. Separately, the Svelte binding also lacks the exponential-backoff auto-reconnect that React and Solid already get from ConnectionManager, so a dropped socket just stays dropped.

Vue has the same shape (it doesn't use ConnectionManager either).

Proposed fix

I think this can be fixed cleanly by extracting and updating work that already exists in-flight:

• The rebuild() primitive on ConnectionManager from the open multi-module-provider PR #4887 (design by @Ludv1gL) — lifted into a focused, additive change and adapted to the current post-#5185 reconnect machinery.
• Porting the Svelte binding onto the shared ConnectionManager (the same pool React and Solid already use), which drops the bespoke singleton, gains auto-reconnect for free, and lets the provider surface a reconnect(builder) method for token swaps with no page reload.

This hangs off the reconnect tracking issue #1936.

Next step

I'll submit a pull request shortly that does both: the ConnectionManager.rebuild() primitive plus tests, then the Svelte port.

Start with ConnectionManager and the Svelte binding provider; review the rebuild() design in PR #4887 and the reconnect machinery added after #5185. Add the proposed rebuild tests, then verify the binding can replace its token and regain exponential-backoff reconnect behavior without a page reload.