CalendarView: CalendarDatePicker alignment and final cleanup

[MartinZikmund]

Phase 8 of CalendarView WinUI 3 parity upgrade

Parent tracking issue: (will be linked from epic)

Scope

Final alignment pass across all files.

Work items

• CalendarDatePicker: Minor error handling alignment, event handler lifecycle cleanup (commented-out DetachHandler calls)
• CalendarView.Properties.cs: Audit for any missing DPs (Uno has 56 public DPs vs WinUI's ~75 generated property accessors — verify no public API gaps)
• Theme XAML template: Remaining template structure differences (navigation button styles, control template adjustments)
• Code cleanup: Remove resolved TODO UNO / UNO-TODO comments (44 occurrences across 16 files). Remove obsolete #if false blocks.
• Enable rounded chrome by default: Flip CalendarViewBaseItemRoundedChromeEnabled to true once all phases validated

Files to modify

• src/Uno.UI/UI/Xaml/Controls/CalendarView/CalendarDatePicker_Partial.cs
• src/Uno.UI/UI/Xaml/Controls/CalendarView/CalendarView.Properties.cs
• src/Uno.UI/UI/Xaml/Controls/CalendarView/CalendarView_themeresources.xaml
• Various files (comment cleanup across all 69 files)

WinUI source reference

Full audit of all CalendarView WinUI source files

Dependencies

All prior phases (1-7)

Risk & Testing

• Risk: LOW - cleanup and polish
• Size: Small-Medium PR (~100-200 lines)
• Testing: Full test suite across all platforms. Visual comparison against WinUI on Windows.