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>

web_interface/templates/v3/partials/general.html

@@ -30,6 +30,10 @@
               showNotification(message, status);
           "
           class="space-y-6">
+        {# Marks this post as the whole form, so an unchecked box saves as
+           false. Without it the save endpoint treats absent keys as unchanged
+           (FORM_SECTION_FIELD in api_v3/config.py). #}
+        <input type="hidden" name="__form_section" value="general">
 
         <!-- Web Display Autostart -->
         <div class="form-group" id="setting-general-web_display_autostart" data-setting-key="web_display_autostart">