[turinglambdaai]

Problem

The Windows quick-add task card already provides its own rounded border, but the nested WinUI TextBox still paints its default blue bottom underline on focus. This makes the input appear to have two competing borders, unlike macOS and Linux.

Change

Override TextControlBorderBrush, TextControlBorderBrushPointerOver, and TextControlBorderBrushFocused locally in the NewTaskBox resource scope to transparent. Keep the outer QuickAddCard border, input behavior, keyboard events, and focus handling unchanged. This intentionally does not affect other text fields (including SearchBox) or globally remove keyboard focus cues.

Verify before merging

• Windows x64 host build and Product Acceptance CI pass
• Visually inspect empty, hovered, focused, typing and unfocused quick-add on Windows 10/11 in light and dark themes
• Keyboard-only tab navigation remains recognizable and Enter continues to submit tasks
• Confirm no horizontal blue line or spacing regressions at 100% and 150% DPI

Standalone UI fix; do not merge/release until cross-platform interaction and installer PRs are validated.