refactor(utils): consolidate size constants and remove unused code

[flexiondotorg]

• Move KB, MB, GB constants to module level for reuse
• Refactor format_size and format_transfer_rate to use shared constants
• Remove unused HTTP_TIMEOUT constant
• Remove unused HashMismatch error variant
• Remove futures dependency and related unused code
• Clean up comments in main.rs and downloader.rs

---

Summary by cubic

Consolidated byte size constants in utils and refactored formatting helpers to use them. Removed unused code and the futures dependency to simplify the codebase and reduce build size; no functional changes.

• Refactors

  • Moved KB/MB/GB constants to module scope in utils; used by format_size and format_transfer_rate.
  • Removed unused HTTP_TIMEOUT constant and HashMismatch error variant.
  • Cleaned up comments in main.rs and downloader.rs.

• Dependencies

  • Dropped futures crate and related code; updated Cargo.toml and Cargo.lock.

^{Written for commit d5a2ce6. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files