Make the inspector actually resize freely

[datlechin]

What

Sets the inspector pane's maximumThickness to NSSplitViewItem.unspecifiedDimension so it can be dragged to any width.

Why

#1676 tried to free the inspector by deleting maximumThickness = 400, but that made it worse. On macOS 14+, NSSplitViewItem(inspectorWithViewController:) defaults both minimumThickness and maximumThickness to 270 ("aren't resizable by default"). Removing the explicit 400 let the max fall back to that 270 default, so min == max == 270: zero divider travel, the inspector was pinned and could not resize at all.

Setting maximumThickness = unspecifiedDimension removes the hard drag stop entirely, which is the documented, necessary-and-sufficient change for unbounded drag width. The 270 minimum stays so the panel keeps a usable floor.

Notes

• recomputeWindowMinSize() only reads minimums, so the window min-size logic is unaffected.
• The divider drag and existing autosaveName ("com.TablePro.mainSplit") persist the chosen width.
• The CHANGELOG entry for free inspector resize already landed in [Unreleased] via Let the inspector panel resize freely #1676, so it's not repeated here; this PR makes that entry true.

Ref: https://developer.apple.com/documentation/appkit/nssplitviewitem/init(inspectorwithviewcontroller:)