Add support for reserved footer area

[diluculo]

This PR resolves #1106 by adding support for a guaranteed minimum space between tabs and footer in TabView. This is particularly critical when using TabView in TitleBar to ensure there's always enough space for DragToMoveArea, regardless of the number of tabs.

Changes:

1. Add reservedStripWidth property to TabView.
2. Modify tab width calculation to consider reserved space:

preferredTabWidth = ((width - 
   (widget.showNewButton ? _kButtonWidth : 0) - 
   (widget.reservedStripWidth ?? 0)) / widget.tabs.length)
   .clamp(widget.minTabWidth, widget.maxTabWidth);

3. Add reserved space in strip layout after new tab button:

// In strip children
if (widget.reservedStripWidth != null)
  SizedBox(width: widget.reservedStripWidth),

Example usage:

TabView(
  footer: const WindowButtons(),
  reservedStripWidth: 100.0,
  stripBuilder: (context, strip) {
    return DragToMoveArea(child: strip);
  },
  // ... other properties
)

Pre-launch Checklist

• I have updated CHANGELOG.md with my changes
• I have run "dart format ." on the project
• I have added/updated relevant documentation

[diluculo]

[WinXaito]

I didn't really understand the goal of this PR, but

to ensure there's always enough space for DragToMoveArea, regardless of the number of tabs.

make total sense with the issue linked.

---

Otherwise LGTM (I didn't test, only take a look at the code). I approved the checks to run.

I let @bdlukaa do the final validation.

[diluculo]

@WinXaito, Thanks for the review! Yes, the main purpose is exactly that - ensuring drag area is preserved regardless of tab count.