feat: add click tracking support

[vklimontovich]

Why

Track clicks on outbound links with full server-side context. Use cases:

• Email campaigns - track which links in newsletters get clicked, tied to user identity
• Social sharing - measure traffic from shared links
• Partner/affiliate links - attribute conversions to specific campaigns
• Documentation - see which external resources users find useful

Unlike client-side click tracking, server-side redirects work in emails and capture requests even when JS is disabled.

Requirements

Middleware Handler

packages/core/src/middleware.ts should handle /api/click/[redirect_url]:

• Configurable route path via config
• Collect serverContext and user info (same as pageView)
• Redirect to destination after tracking

URL Handling

The [redirect_url] should support:

• URL-encoded URLs
• Base64-encoded URLs
• Auto-detect format

New Types

Add ClickContext type:

export interface ClickContext {
  destination: string;
}

GA Backend

Update GA backend to handle click events appropriately.

Notes

• Use after() for non-blocking tracking before redirect
• Handle invalid URLs gracefully