fix(schedule): stop stray 'days' data from overriding Global schedule

save_schedule_config never persisted the schedule's 'mode' field, and
_check_schedule inferred per-day vs global purely from whether a 'days'
dict was present for the current day. Config migration
(_merge_template_defaults) re-adds the template's 'schedule.days' (all
days disabled by default) whenever it's missing from the user's saved
config - which is exactly the case after saving Global mode, since that
save path intentionally pops 'days'. The result: a user on Global mode
would get their schedule silently reinterpreted as per-day, with today's
day disabled, blanking the display.

Persist 'mode' on save and have _check_schedule honor it explicitly
(mirroring how _check_dim_schedule already does), so a resurrected
'days' dict can't override an explicit Global selection. Falls back to
the old inference behavior only when no 'mode' is recorded.

web_interface/blueprints/api_v3.py

@@ -329,6 +329,7 @@ def save_schedule_config():
         }
 
         mode = data.get('mode', 'global')
+        schedule_config['mode'] = mode
 
         if mode == 'global':
             # Simple global schedule