fix(datagrid): render sort chevron via SF Symbol with palette tint

[datlechin]

Summary

Fixes a Dark mode regression introduced by #945. The sort chevron was invisible in Dark mode (visible in Light mode).

Root cause

SortableHeaderCell.drawInterior used the private AppKit SPI images NSAscendingSortIndicator / NSDescendingSortIndicator. These are template images. The helper function drawTintedIndicator was misleadingly named — it didn't actually tint anything, it just called image.draw(in:...). Template images then rendered in the default graphics-context fill color, which is BLACK.

• Light mode: black chevron on light header background → visible
• Dark mode: black chevron on dark header background → invisible

Fix

Switch to SF Symbols (chevron.up / chevron.down) configured with SymbolConfiguration.paletteColors([.secondaryLabelColor]). SF Symbol palette tinting is appearance-aware, so Light/Dark mode rendering just works.

Renamed the helper from drawTintedIndicator to drawIndicator since it no longer does any tinting (the symbol is pre-tinted via configuration). Function name now matches what it does.

Why this is the cleanest approach

• SF Symbols are public API — stable across macOS releases
• SymbolConfiguration.paletteColors tints at the symbol-rendering layer, respecting current NSAppearance automatically
• No manual NSGraphicsContext save/restore + .sourceIn blending hacks
• Multi-sort rendering unchanged (priority number drawn next to chevron via separate text path)

Test plan

• Light mode: click column header → chevron visible
• Dark mode: click column header → chevron visible (was missing before this fix)
• Toggle Light ↔ Dark mid-session: chevron tint updates
• Multi-sort: shift-click second column → both columns show chevrons, secondary has priority number "2" / "3" alongside
• `xcodebuild build` clean
• `swiftlint lint --strict` 0 violations