feat: add support for configurable first day of the week in calendar

[Cprakhar]

Description

Interpret clock.calendar.first-day-of-week integers as 0–6 with 0 = Sunday.
Log a warning if no correct value is provided, and fallback to the original logic.

Verification

Configuration

"clock": {
        "interval": 1,
        "format": "󰥔 {:%H:%M}",
        "timezone": "Asia/Kolkata",
        "locale": "en_IN.UTF-8",
        "tooltip": true,
        "tooltip-format": "<tt>{calendar}</tt>",
        "format-alt": "󰃭 {:%A, %B %d %Y}",
        "calendar": {
            "mode": "month",
            "mode-mon-col": 4,
            "on-scroll": -1,
            "first-day-of-week": 2,
            "format": {
                "months": "<span color='#fabd2f'><b>{}</b></span>",
                "days": "<span color='#83a598'><b>{}</b></span>",
                "today": "<span color='#fe8019'><b>{}</b></span>",
                "weekdays": "<span color='#8ec07c'><b>{}</b></span>"
            }
        },
        "actions": {
            "on-click-right": "mode",
            "on-scroll-up": "shift_up",
            "on-scroll-down": "shift_down"
        }
    }

Closes #4771

[Alexays]

Can you add this new option to the man? 🙏

[Cprakhar]

@Alexays, I have updated the man page.