Switch calendars to Sunday-first layout

[multiplex55]

Motivation

• Align calendar month grids to a Sunday-first convention so months that start on Sunday appear in the first column.
• Make the popover and dashboard calendar widgets display weekdays in the same order for consistency.
• Ensure visual header labels match the offset used to populate the grid.

Description

• Replaced month.weekday().num_days_from_monday() with month.weekday().num_days_from_sunday() in src/gui/calendar_popover.rs and src/dashboard/widgets/calendar.rs to compute the grid offset.
• Reordered weekday header arrays to Sun, Mon, Tue, Wed, Thu, Fri, Sat in both src/gui/calendar_popover.rs and src/dashboard/widgets/calendar.rs.
• Left the existing grid population logic (42-cell grid, spacing, selectable labels and markers) unchanged so behavior aside from ordering remains the same.

Testing

• No automated tests were run on these changes.

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Codex Task