feat: customizable start/end labels for range time input

[balajis-qb]

Summary

Fixes #6280.

When selectsRange is combined with showTimeInput, the calendar renders two time inputs and hardcodes English (Start) / (End) suffixes onto timeInputLabel (calendar.tsx), with no way to override or translate them — a gap for non-English apps.

This adds two optional props:

Prop	Effect
timeInputStartLabel	When set, replaces the entire caption of the start time input
timeInputEndLabel	When set, replaces the entire caption of the end time input

When a prop is not set, the existing "<timeInputLabel> (Start)" / " (End)" behaviour is unchanged. Single (non-range) showTimeInput mode is unaffected.

<DatePicker
  selectsRange
  showTimeInput
  timeInputStartLabel="Startzeit"
  timeInputEndLabel="Endzeit"
  /* ... */
/>

Changes

• src/calendar.tsx — declare timeInputStartLabel / timeInputEndLabel on CalendarProps (they flow to DatePickerProps automatically); use them in renderInputTimeSection.
• src/test/datepicker_test.test.tsx — 6 tests: default suffixes, back-compat with timeInputLabel, full-caption replacement, precedence over timeInputLabel, per-side fallback, single-mode unaffected.
• docs-site/ — new "Input Time with Range (custom start/end labels)" example.

Screenshots

Default Mode

Override Lables

Contribution checklist

• I have followed the contributing guidelines.
• I have added sufficient test coverage for my changes.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (548a1f3) to head (cc108fb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6325      +/-   ##
==========================================
+ Coverage   99.29%   99.32%   +0.02%     
==========================================
  Files          30       30              
  Lines        3822     3824       +2     
  Branches     1648     1650       +2     
==========================================
+ Hits         3795     3798       +3     
+ Misses         26       25       -1     
  Partials        1        1              

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[balajis-qb]

Hi @martijnrusschen, Can you review this PR and let me know if any changes required. Thank you.