refactor(date-picker): comprehensive cleanup and reliable crash fix

The previous fix still had derived Date objects in useEffect deps.
Object.is(new Date(), new Date()) === false, so any Date in deps causes
the effect to run every render, reproducing the infinite loop on
re-open with existing time selection.

Key changes:
- useEffect deps now use only stable primitives (startDate, endDate strings)
  and compute Date values inside the effect — eliminating the loop
- Replace `rest as any` with a FlatDatePickerProps merged type for safe,
  typed destructuring across the discriminated union
- Remove initialStart/initialEnd render-scope variables; compute inline or
  inside effects to keep derivation local to each use site
- Callbacks use destructured props (onChange, onRangeChange, etc.) instead
  of props.x references
- Remove verbose TSDoc on internal callbacks — names are self-documenting
- Preserve all existing JSX structure and CalendarMonth logic unchanged

Author: waleedlatif1