fix(web): plugin settings form shows schema defaults for unsaved keys (#597)

The server-rendered plugin settings partial rendered straight from the
saved config, so an option added in a plugin update (geochron 1.2.0's
show_date / show_date_line, default true) drew as an unchecked box, and
the save route's missing-checkbox handling then stored it as false.
Enum dropdowns likewise showed their first option instead of the default.

- _load_plugin_config_partial runs the stored section through
  prepare_plugin_config (as GET /plugins/config does) before masking
  secrets, so a secret's schema default is masked too.
- render_field falls back to the field's own default, covering children
  of objects that declare a default of their own (where the defaults
  extraction stops).
- The legacy-boolean parity test now compares against the config the
  plugin actually runs with (defaults included).

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

web_interface/templates/v3/partials/plugin_config.html

@@ -29,6 +29,11 @@
 
 {% macro render_field(key, prop, value, prefix='', plugin_id='') %}
     {% if not prop_is_hidden(prop)|trim %}
+    {# A key the saved config doesn't have renders its schema default. The
+       route merges defaults first, but that merge stops at an object that
+       declares its own default (often {}), leaving its children missing here.
+       Booleans matter most: an unchecked box saves as false. #}
+    {% if value is none and prop.default is defined %}{% set value = prop.default %}{% endif %}
     {% set full_key = (prefix ~ '.' ~ key) if prefix else key %}
     {% set field_id = (plugin_id ~ '-' ~ full_key)|replace('.', '-')|replace('_', '-') %}
     {% set label = prop.title if prop.title else key|replace('_', ' ')|title %}