fix(web): partial JSON saves to /config/main change only what they send

A JSON body with one field reset every checkbox in the sections it touched:
the MQTT bridge's brightness slider turned off disable_hardware_pulsing,
inverse_colors, show_refresh_rate and use_short_date_format, and a
timezone-only save turned off web-UI autostart and weekly auto-updates.
Missing-means-unchecked now applies only to form posts: form-encoded bodies
and the v3 forms, which mark themselves with a hidden __form_section input.

Also on the config routes:
- vegas_min/max_cycle_duration no longer match the generic *_duration rule,
  so they stop landing in display_durations and a blank one no longer
  rejects the whole Display save;
- saving from the Raw JSON editor calls start_setup_if_needed like the
  General form, so enabling auto-update there finishes its setup;
- the schedule and dim-schedule POSTs accept the per-day days.<day> shape
  their GETs return, as well as the flat form keys.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_auto_update.py

@@ -668,7 +668,9 @@ class TestSettingsSave:
 
     def test_unchecked_toggle_on_general_save_disables(self, api_client):
         client, cm, setup_calls = api_client
-        resp = client.post('/api/v3/config/main', json={'timezone': 'UTC'})
+        # The General form marks its posts; an unchecked box is then absent.
+        resp = client.post('/api/v3/config/main',
+                           json={'__form_section': 'general', 'timezone': 'UTC'})
         assert resp.status_code == 200
         assert self._saved(cm)['auto_update'] == {'enabled': False}
         assert setup_calls == [True]