perf(widgets): avoid re-splitting lines on every build in VirtualSelectableTextView #1532

Problem

In VirtualSelectableTextView.build (lib/core/widgets/virtual_selectable_text_view.dart), final lines = text.split('\n'); is executed unconditionally on every widget build.
During window resize, theme change, focus shift, or parent animations, a multi-thousand-line string is re-split into a List<String> on every single frame, causing UI stutter.

Solution

• Convert to a StatefulWidget or compute/cache line offsets/slices (LineIndex) only when widget.text changes (didUpdateWidget).
• Avoid running text.split('\n') inside build().

Benefits

• Zero string splitting during re-renders, layout recalculations, and theme changes.
• Stable 60/120 FPS during window resizing with large text views.