feat: honor block size environment

[ShivamB25]

Summary

• honor BLOCK_SIZE, falling back to BLOCKSIZE, across stdout, TUI, and export-progress terminal formatting
• support fixed GNU-style byte block sizes, human-readable, and si; round positive block counts up
• preserve precedence for explicit size-format flags and keep exported JSON values in raw bytes
• make formatter tests deterministic under externally supplied block-size variables

Verification

• go test -v -covermode=count ./...
• env BLOCK_SIZE=1K go test ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.11.2 run ./internal/common/... ./stdout/... ./tui/... ./report/... ./cmd/gdu/app/... ./cmd/gdu/...
• CLI smoke tests with BLOCK_SIZE=1K and BLOCKSIZE=kB

Closes #120

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.89%. Comparing base (1868609) to head (48273fe).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
+ Coverage   85.69%   85.89%   +0.19%     
==========================================
  Files          55       55              
  Lines        6243     6329      +86     
==========================================
+ Hits         5350     5436      +86     
  Misses        689      689              
  Partials      204      204              

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