Add Stadia tile proxy with caching and theme attribution fixes

[fityannugroho]

PR Checklist

Please check if your PR fulfills the following requirements:

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• I have read the documentation.
• I have read and followed the Contributing Guidelines.
• I have included a pull request description of my changes.
• I have included the necessary changes to the documentation.
• I have added tests to cover my changes.

PR Type

What kind of change does this PR introduce?

Please check any kind of changes that applies to this PR using [x]

• Bug fix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• ..... (describe the other type)

What is the current behavior?

Please describe the current behavior that you are modifying, or link to a relevant issue.

Issue Number: N/A

• Stadia tiles used to work without an API key, but now require a key / enforce stricter limits; fetching directly from the client can’t keep the key server-side.

What is the new behavior?

• Added /api/stadia-tiles/[...path] proxy route that:
  • Awaits route params (Next.js 15 compatible)
  • Injects STADIA_API_KEY server-side
  • Adds Cache-Control + Next.js revalidate with configurable TTL/SWR
• Client style loading now optionally rewrites Stadia tile URLs to the proxy when NEXT_PUBLIC_STADIA_TILE_PROXY=1 (works in any environment).
• Theme switches recreate the Maplibre layer to ensure attribution updates correctly, with cleanup to prevent duplicates and abort-safe style fetching.
• Referrer policy is set via next.config.js and app/layout.tsx.
• .env.example documents the new env vars.

Other information

• Env:
  • STADIA_API_KEY
  • NEXT_PUBLIC_STADIA_TILE_PROXY
  • TILE_CACHE_MAX_AGE
  • TILE_CACHE_STALE_WHILE_REVALIDATE