test(frontend): make date assertions machine-independent

Three tests encoded the machine's timezone or locale into their
expectations and fail on any box east of UTC (observed on UTC+12/13;
CI's UTC and US-locale dev machines never see it):

- dateTimeUtils isSame: compared 10:00Z/11:00Z on the 15th as 'same
  day', but isSame works on local calendar days and no UTC instant
  falls on the same date in every timezone (at UTC+13 those are 23:00
  on the 15th and 00:00 on the 16th). Use timezone-less datetimes,
  which mean the same calendar day everywhere.
- RecordingUtils toDateString: formatted a UTC-noon instant and
  expected the UTC date, but toDateString renders local time (UTC+12
  renders the next day). Construct the Date with local components.
- SeriesModalUtils getEpisodeAirdate: the code renders the viewer's
  locale via toLocaleDateString() by design, but the test hardcoded
  the en-US shape (it tolerated the timezone day-shift with /1[4|5]/
  yet not the day/month order). Compute the expectation with the same
  API so the test asserts the wiring, not a specific locale.

No production code changes.

frontend/src/utils/__tests__/dateTimeUtils.test.js

@@ -132,14 +132,14 @@ describe('dateTimeUtils', () => {
 
   describe('isSame', () => {
     it('should return true when dates are same day', () => {
-      const date1 = '2024-01-15T10:00:00Z';
-      const date2 = '2024-01-15T11:00:00Z';
+      const date1 = '2024-01-15 10:00:00';
+      const date2 = '2024-01-15 11:00:00';
       expect(dateTimeUtils.isSame(date1, date2)).toBe(true);
     });
 
     it('should return false when dates are different days', () => {
-      const date1 = '2024-01-15T10:00:00Z';
-      const date2 = '2024-01-16T10:00:00Z';
+      const date1 = '2024-01-15 10:00:00';
+      const date2 = '2024-01-16 10:00:00';
       expect(dateTimeUtils.isSame(date1, date2)).toBe(false);
     });
 

frontend/src/utils/components/__tests__/SeriesModalUtils.test.js

@@ -353,7 +353,7 @@ describe('SeriesModalUtils', () => {
     it('should format valid air date', () => {
       const episode = { air_date: '2024-01-15' };
       const formatted = getEpisodeAirdate(episode);
-      expect(formatted).toMatch(/1\/1[4|5]\/2024/);
+      expect(formatted).toBe(new Date('2024-01-15').toLocaleDateString());
     });
 
     it('should return N/A for missing air date', () => {

frontend/src/utils/forms/__tests__/RecordingUtils.test.js

@@ -111,7 +111,7 @@ describe('RecordingUtils', () => {
 
   describe('toDateString', () => {
     it('formats a Date to YYYY-MM-DD', () => {
-      const d = new Date('2024-06-15T12:00:00Z');
+      const d = new Date(2024, 5, 15, 12, 0, 0);
       const result = toDateString(d);
       expect(result).toBe('2024-06-15');
     });