feat: auto-select US or EU based on cloud region

[vmaineng]

Closes #15894

Changes

Auto-selects the US/EU code snippet tabs based on the user's cloud region, so users no longer have to manually switch tabs when following docs.

After digging into the codebase, PostHog already has feature flags (direct-to-eu-cloud / direct-to-us-cloud) that determine a user's cloud region. This PR wires those existing flags into the tab selection logic in two places:

• src/components/CodeBlock/index.tsx — when direct-to-eu-cloud is enabled, the EU tab is now auto-selected in addition to the existing region string replacement that was already happening

• src/components/Tab/index.tsx — added useCloud() hook so generic US/EU tabs also auto-select based on cloud region, with the correct priority chain:

  1. URL ?tab= param (highest priority, preserves shareable links)
  2. Cloud region from feature flag
  3. Default to first tab (US)
  

Testing

Tested locally by temporarily setting the direct-to-eu-cloud condition to true in CodeBlock and confirming the EU tab auto-selects on pages with file=US / file=EU code blocks (e.g. /docs/advanced/proxy/nextjs).

• [X ] I've read the docs and/or content style guides.
• [ X] Words are spelled using American English
• Use relative URLs for internal links
• I've checked the pages added or changed in the Vercel preview build
• If I moved a page, I added a redirect in vercel.json