Replace Deprecated Flutter APIs

[fcasco]

Description

Replace all deprecated Flutter APIs with their modern equivalents to ensure compatibility with the latest Flutter SDK.

Tasks

• Replace accentColor with colorScheme.secondary
• Replace textTheme.caption with textTheme.bodySmall
• Fix textSelectionHandleColor issues (marked with FIXME comments)
• Update any other deprecated APIs found during testing
• Ensure theme definitions follow Material 3 guidelines

Priority

Medium