fix(frontend): prevent RangeError in TimeAdjuster and add validation to SleepButton

frontend/tests/unit/time.test.ts

@@ -12,6 +12,7 @@ import {
   adjustForTimezone,
   getLocalOffsetFromReference,
   formatDateTime,
+  toUTCFromLocalInput,
 } from '../../src/utils/time';
 
 describe('formatMinutesAfter', () => {
@@ -152,3 +153,18 @@ describe('getLocalOffsetFromReference', () => {
     expect(offset).toBe(localMins - refMins);
   });
 });
+
+describe('toUTCFromLocalInput', () => {
+  it('converts valid datetime-local string to ISO UTC string', () => {
+    const input = '2024-01-15T10:00';
+    // This is tricky because it depends on local timezone. 
+    // But we just want to see if it works for a valid input.
+    const result = toUTCFromLocalInput(input);
+    expect(result).toMatch(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3}Z$/);
+  });
+
+  it('returns empty string on invalid datetime-local string', () => {
+    expect(toUTCFromLocalInput('2026-06-0')).toBe('');
+  });
+});
+