fix: align data grid header padding

[overtrue]

Summary

• Add the same 4pt horizontal inset to data grid column headers that result cells use.
• Render the row-number # header with the same header cell and right alignment as row-number cells.
• Add coverage for SortableHeaderCell.titleRect(forBounds:).

before

after

Root Cause

Data cells applied 4pt leading and trailing constraints, but unsorted headers used the default NSTableHeaderCell interior drawing. That left header titles flush against grid edges, and the row-number header used a different cell path from the row-number content cells.

Validation

• xcodebuild -project TablePro.xcodeproj -scheme TablePro test -skipPackagePluginValidation -only-testing:TableProTests/SortableHeaderCellTests CODE_SIGNING_ALLOWED=NO
• swiftlint lint --strict --config .swiftlint.yml using the SwiftLint plugin binary
• xcodebuild -project TablePro.xcodeproj -scheme TablePro -configuration Debug build -skipPackagePluginValidation CODE_SIGNING_ALLOWED=NO

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[Copilot]

Pull request overview

This PR fixes a visual inconsistency in the Results data grid by ensuring table column headers use the same horizontal padding as result cells, and by rendering the row-number # header using the same custom header cell path as other headers (including right alignment). It also adds targeted unit coverage for the header title inset logic.

Changes:

• Replace default unsorted header interior drawing with custom title drawing that applies consistent horizontal insets.
• Use SortableHeaderCell for the row-number (#) header and align it to the right to match row-number cells.
• Add tests validating SortableHeaderCell.titleRect(forBounds:) padding behavior and clamping for narrow widths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
TableProTests/Views/Results/SortableHeaderCellTests.swift	Adds tests verifying header title rect padding and non-negative width behavior.
TablePro/Views/Results/SortableHeaderCell.swift	Implements padded titleRect(forBounds:) and unifies title drawing for sorted/unsorted headers.
TablePro/Views/Results/DataGridView.swift	Switches the row-number column header to use SortableHeaderCell with right alignment.
CHANGELOG.md	Documents the header padding fix in the “Fixed” section.

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[datlechin]

LGTM, thank you