🩹 fix crash on updatePrefReminderTime

[JGreenlee]

If user updates the "Time of Day for Reminders", it will store as "Invalid DateTime" and cause a crash.

newTime here is not an ISO string; it is a JavaScript Date object. Attempting to parse it as an ISO string results in "Invalid DateTime" being stored to the user prefs and a crash on the Profile screen

This fix ensures that the selected time will be stored, and "Invalid DateTime" will not get stored.

But if any user had already encountered this, and "Invalid DateTime" was already stored to their profile, they would still not be able to access the Profile screen without it crashing.

So the second change makes sure notifScheduler only attempts to schedule notifications if the datetime is valid.
If not valid, user will see this popup:

After dismissing the popup, they can change their "Time of Day for Reminders" and it will set correctly this time

[JGreenlee]

@shankari for merge
@sebastianbarry for visibility

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (dcb6957) 78.45% compared to head (e84e816) 78.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1130      +/-   ##
==========================================
- Coverage   78.45%   78.37%   -0.08%     
==========================================
  Files          28       28              
  Lines        1699     1702       +3     
  Branches      364      366       +2     
==========================================
+ Hits         1333     1334       +1     
- Misses        366      368       +2     

Flag	Coverage Δ
unit	78.37% <50.00%> (-0.08%)	⬇️

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Files	Coverage Δ
www/js/splash/notifScheduler.ts	98.47% <50.00%> (-1.53%)	⬇️