Fix day dropdowns in price config to respect selected month

[Copilot]

The start/end day dropdowns (rsd/red) in price modifier configuration always offered 1–31 regardless of the selected month, allowing nonsensical dates like Feb 31.

Changes

• daysInMonth(month) — computes correct day count per month; uses 2024 (leap year) as reference so Feb correctly offers up to 29 days. Returns 31 when no month is selected (value 0).
• clampDay(dateObj, month) — clamps the bound day value to the new month's max on month change, preventing stale out-of-range selections.
• rsd/red dropdowns — {length: 31} replaced with {length: daysInMonth(c.s.m)} and {length: daysInMonth(c.e.m)} respectively.
• rsm/rem selects — on:change handlers call clampDay so switching e.g. from March (day 31) to February automatically resets the day to 29.

function daysInMonth(month) {
    if (month === 0) return 31;
    return new Date(2024, month, 0).getDate();
}

function clampDay(dateObj, month) {
    const max = daysInMonth(month);
    if (dateObj.d > max) dateObj.d = max;
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Days in price configuration are not month-dependent</issue_title>
<issue_description>Describe the bug
When choosing time-dependent fees in price configuration, the days dropdown always goes from 1 to 31, independently of the month chosen.

To Reproduce
Steps to reproduce the behavior:

1. Go to Settings
2. Click on Configure Price
3. Scroll down to any modifier
4. Click on the dropdown for days
5. The array goes from 1 to 31

Expected behavior
I expect that the available days to choose depends on the month selected. I don't know what would happen if I selected e.g. Feb 31.

Screenshots

Hardware information:

• Country: Denmark
• Meter: Kamstrup
• Encryption enabled [yes/no]
• AMS reader: Pow-K+
• M-bus adapter (if applicable):

Relevant firmware information:

• Version: v2.5.6
• MQTT: yes
• MQTT payload type: home-assistant

</issue_description>

<agent_instructions>The file is PriceConfig.svelte. The dropdowns named rsd and red are always showing 31 days, regardless of month. Please suggest solution where the maximum day is limited to the selected month in rsm and rem respectively</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Days in price configuration are not month-dependent #1165

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