fix(night-light): warn when schedule start equals end

From review: equal start/end silently disables the tint (zero-length window).
Show an inline hint so the user isn't left wondering why nothing happens.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/settings/general/General.tsx

@@ -634,6 +634,11 @@ function Appearance() {
                 </Box>
               </Box>
             )}
+            {nightLightSchedule && nightLightStart === nightLightEnd && (
+              <Text size="T200" style={{ color: color.Critical.Main }}>
+                Start and end times are the same, so the tint never turns on. Set different times.
+              </Text>
+            )}
           </Box>
         )}
       </SequenceCard>