Hardcoded Cloudflare R2 public URL prevents custom bucket configuration

[vedantterse]

The Cloudflare R2 public URL is hardcoded in multiple files, preventing users from configuring their own R2 buckets. This makes the image upload functionality non-functional for anyone cloning the repository.

Affected files:

upload-image.ts (line 26)

next.config.ts (line 13)

Details:
Since the R2 public URL is hardcoded, users need to manually find and replace it in the codebase for uploads to work. This leads to unnecessary setup errors and makes the starter less beginner-friendly.

Expected behavior:
The R2 public URL should be configurable via an environment variable (e.g. R2_PUBLIC_URL) so that custom buckets work out of the box.