Range calendar jalali onchange undefined problem

[sinabdollahi]

This fixes #246

• dateToUnix now returns NaN for empty values instead of “now”.
• Prevents false comparisons when no start/end exists yet.
• isDateBetween now uses numeric timestamps explicitly.
• Removes fragile object coercion behavior and keeps comparisons deterministic.
• removeTime now normalizes from an existing parsed date and only applies timezone when needed.
• Avoids double/ambiguous parsing.
• days.tsx now checks startDate && endDate before “same-day range” comparison.
• Prevents accidentally evaluating dayjs(undefined) and collapsing range styling.
• onChange in range mode now converts already-normalized dates directly.

- So in short: it fixed because we made the range pipeline strictly compare real dates only, never “fallback now”, and avoid double parsing — which was especially visible in Jalali mode.

[farhoudshapouran]

Thanks @sinabdollahi — I reviewed the changes carefully, and they all look good:

✅ dateToUnix → NaN — correctly fixes the silent "fallback to now" issue that caused the reset-on-first-tap symptom
✅ isDateBetween numeric comparison — safer and deterministic
✅ removeTime conditional .tz() — good defensive change
✅ days.tsx startDate/endDate guard — fixes a real range-highlight bug
✅ Range onChange simplification — verified Gregorian + timezone still emits correct dates

Merging. Thanks for the careful work and the clear description.

While testing, I found that calendar="jalali" + timeZone="Asia/Tehran" + mode="range" still crashes with RangeError: Invalid time value and emits dates with year 1405 AD. This bug exists on main too (pre-existing), so this PR doesn't introduce it.

Root cause: jalali-plugin-dayjs mutates global dayjs state and interacts badly with dayjs.tz() + .startOf('day').

I'll open a separate issue to track the Jalali + timezone fix. It likely requires a library-level change (replacing the plugin).