feat: replace date selection with calendar picker in share dialogs

[Copilot]

Replaces the text-based date input field with a visual calendar picker in share dialogs, providing a more intuitive and error-free date selection experience.

Changes

QML Interface

• New Component: Added NCCalendarPicker.qml with full calendar functionality
• Visual Calendar Grid: 7-column weekday layout with proper day headers
• Month Navigation: Previous/Next arrows for browsing months
• Interactive Selection: Click-to-select dates with visual feedback
• Date Constraints: Respects minimum/maximum date limits from backend
• Today Highlighting: Special styling for current date
• Quick Actions: Today button and Cancel option
• Nextcloud Styling: Uses Style.ncBlue accent color for consistency

Apple VFS Integration

Enhanced ShareOptionsView.swift to use improved calendar presentation:

• Changed NSDatePicker to .clockAndCalendar style
• Added .yearMonth and .yearMonthDay elements for comprehensive date selection
• Applied consistent calendar styling across all date picker instances

Implementation Details

• API Compatibility: Maintains identical interface with existing NCInputDateField
• Zero Breaking Changes: Drop-in replacement in ShareDetailsPage.qml
• DateFieldBackend Integration: Leverages existing C++ backend for date handling
• Qt 6.8 Compatible: Uses standard QtQuick components without external dependencies

User Experience Improvements

Before: Users had to manually type dates in a text field, prone to format errors and locale-dependent formatting issues.

After: Users see a visual calendar interface with:

• Clickable date grid with clear month/year display
• Visual feedback for today, selected dates, and invalid dates
• Intuitive month navigation
• One-click "Today" selection for convenience
• Proper handling of date constraints (minimum/maximum dates)

Testing

The calendar picker respects all existing date validation rules and integrates seamlessly with the existing share dialog workflow. The Swift implementation provides an enhanced calendar experience for Apple VFS users.

Fixes the date selection usability issues by providing modern calendar picker interfaces for both QML screens (using Qt 6.8 capabilities) and Apple VFS integration.

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