fix(notifications): flag inactive quiet-hours when a time field is empty

Clearing the quiet-hours start or end time silently deactivated the window
(isWithinTimeWindow → parseHHMM('') is null → returns false) while the toggle
still read "on", with no indication. Added an inline Critical-colored hint —
"Set both a start and end time — quiet hours stay inactive until both are filled
in" — shown when the toggle is on but either field is empty. Non-destructive:
it explains why rather than guessing a default time. Copy verified against
isWithinTimeWindow.

Last pure-client bug-hunt finding from LOTUS_TODO (the rest are live-call /
desktop-gated). Gate-green (tsc, eslint, prettier, build).

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

src/app/features/settings/notifications/SystemNotification.tsx

@@ -381,6 +381,11 @@ export function SystemNotification() {
                 style={selectStyle}
               />
             </Box>
+            {(!quietHoursStart || !quietHoursEnd) && (
+              <Text size="T200" style={{ color: color.Critical.Main }}>
+                Set both a start and end time — quiet hours stay inactive until both are filled in.
+              </Text>
+            )}
           </Box>
         )}
       </SequenceCard>