fix: prevent crash when date props are passed as strings

[martijnrusschen]

Summary

• Added safeToDate() helper function in date_utils.ts that validates date values at runtime
• Returns null for invalid inputs (strings, numbers, invalid Date objects), allowing graceful fallback
• Fixes crash with "getTime/getFullYear is not a function" when date props are passed as strings

Fixed Locations

• time.tsx: isSelectedTime(), renderTimes() - validate selected/openToDate props
• index.tsx: handleTimeOnlyArrowKey(), handleTimeOnlyInputKeyDown() - validate selected prop
• index.tsx: handleInputChange() - validate startDate/endDate before calling .getTime()

Root Cause

TypeScript types selected, startDate, endDate, etc. as Date | null, but at runtime JavaScript consumers or incorrect usage could pass strings. The code assumed these were always valid Date objects and called methods like .getTime() directly, causing crashes.

Solution

The safeToDate() function checks:

1. If the value is null or undefined → returns null
2. If the value is a valid Date object (using isDate() and isValid() from date-fns) → returns the date
3. Otherwise → returns null

This allows the existing fallback chains (e.g., selected || openToDate || newDate()) to work correctly.

Test Plan

• Added 11 unit tests for safeToDate() function covering valid dates, null, undefined, strings, numbers, invalid Date objects
• Added 5 integration tests for TimePicker with string date props
• All 1464 tests pass
• Linting passes

Fixes #5964

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.26%. Comparing base (be38873) to head (149d547).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6166   +/-   ##
=======================================
  Coverage   99.26%   99.26%           
=======================================
  Files          30       30           
  Lines        3792     3801    +9     
  Branches     1631     1635    +4     
=======================================
+ Hits         3764     3773    +9     
  Misses         27       27           
  Partials        1        1           

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