refactor: delete package c2h5oh/datasize and create human readable format

[updevru]

Refactor: replace external size-formatting dependency with internal utilities.

• Removed dependency on github.com/c2h5oh/datasize.
• Added utilities in util/bytes.go:
  • String formatting of bytes using binary units with integer output: ByteToHumanReadable(uint64) → "KiB/MiB/GiB/TiB/PiB/EiB".
  • Conversions from bytes to binary units with fractional precision: ByteToKBytes/…/ByteToEBytes(int64) → float64.
• Updated usages:
  • frac/common/info.go: sizes are printed via util.ByteToHumanReadable.
  • storeapi/docs_stream.go and util/log_helpers.go: sizes in logs via util.ByteToHumanReadable.
  • util/util.go: SizeToUnit now uses the new converters; removed SizeStr.
• Added tests util/bytes_test.go covering:
  • Basic cases, thresholds, ±1 around thresholds.
  • Very large values (including MaxUint64) without overflows.
• String output now explicitly uses binary units (KiB/MiB/…), removing ambiguity between binary thresholds and decimal suffixes.
• Avoided uint64 → int64 casts in formatting to prevent overflows.

Motivation:

• Reduce third-party dependencies.
• Clear, consistent unit semantics (binary units and thresholds).
• Controlled formatting for logs/strings and predictable conversions for metrics.

Affected areas:

• Logging and string formatting of sizes.
• Helper utilities for size conversions.
• Unit tests for formatting.

Backwards compatibility:

• Log string format changed: now using KiB/MiB/… suffixes with integer values (no fractional part). If fractional precision is needed in strings, this can be added later without breaking the current API.

Dependencies

• Removed: github.com/c2h5oh/datasize
• Used: github.com/alecthomas/units (directly in util/bytes.go)

Fixes #146

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