perf(ui): flyweight Paint and zero-allocation line drawing in QueryaTreeIndentGuide #1526

Problem

In _QueryaTreeIndentGuidePainter.paint (lib/core/ui/querya_tree_indent_guide.dart), a new Paint() object is instantiated on every paint call, and canvas.drawLine(Offset(x, 0), Offset(x, size.height), paint) is invoked in a loop per depth level.
At 60 FPS during sidebar scroll (50 visible items * depth 3), this generates ~21,000 objects per second, causing GC pressure during scroll animations.

Solution

• Use a reusable static/field Paint instance with updated color/strokeWidth.
• Minimize Offset allocations (or cache point pairs where possible).

Benefits

• Zero Paint object allocations during tree scrolling.
• Smooth 60/120 FPS sidebar navigation even in deep database trees.