Fix null reference in ChipGroup, empty catch block in SingleLineInputField, hardcoded color in FloatingNavigationButton

[Copilot]

Description of Change

Project review identified three issues:

• ChipGroup.cs — FirstOrDefault() result accessed via ! without null guard in both Single and Multi selection paths of SetChipsToggledBasedOnSelectedItems. Crashes when SelectedItems contains values not present in m_chipItems.

// Before: NullReferenceException if no matching chip
var chipItem = m_chipItems.FirstOrDefault(...);
chipItem!.Chip.IsToggled = true;

// After
var chipItem = m_chipItems.FirstOrDefault(...);
if (chipItem is null) return;
chipItem.Chip.IsToggled = true;

• SingleLineInputField.cs — Empty catch (Exception e) {} in Focus() silently swallowed all errors. Now logs via DUILogService.LogError, consistent with the rest of the codebase.

• FloatingNavigationButton.cs — Hardcoded Color.FromArgb("#BF8DCE") in two locations replaced with Colors.GetColor(ColorName.color_fill_highlight). Restores design system compliance and dark mode support.

Todos

• I have tested on an Android device.
• I have tested on an iOS device.
• I have supported accessibility

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