Add subprotocol check to the client

[ilyatitovich]

Fixes #245

• Added isClientUpdateRequired atom store to trigger Update Client page when the client uses an unsupported subprotocol (below server minSubprotocol);
• Added handling of unsupported subprotocols for both HTTP requests and Logux WebSocket errors;
• Added router logic to redirect users to /update-client when a client update is required;
• Added a simple Update Client page with temp test data;
• Added tests.

Motivation

This change introduces a unified mechanism for detecting unsupported client versions across HTTP and Logux, and redirects users to an update page.

[ai]

Pretty solid PR (expect page design, to be honest). All changes are coming from things missed in the task. Good work 👍

[ilyatitovich]

• Removed custom headers. Now outdated client detection uses response body;
• Redesign Update Client page.

I'm a bit confused about router. Now with:

if (outdatedClient) {
   nextRoute = redirect(open('updateClient'))
}

we redirect user to Update Client page. Should we keep this behavior or just use nextRoute = open('updateClient')?

In main.svelte we pass $currentPage into component, so I assume that udpateClient must be set in core router.

[ilyatitovich]

• Removed handleOutdatedClient helper;
• Removed isOutdatedClient from the core router: the Update Client page is now handled by the web router.