feat(web): x-advanced schema flag groups plugin config fields under a collapsed Advanced Settings section

Plugin config pages show every schema property at equal visual priority,
which overwhelms first-time users. Plugin authors can now add
"x-advanced": true to any flat (non-object) property in config_schema.json
to move it into one collapsed "Advanced Settings (N)" section rendered after
the basic fields - progressive disclosure with zero loss of control.

Implementation: the main render loop in plugin_config.html splits ordered
properties into basic/advanced tiers; the advanced group reuses the exact
.nested-section/.nested-content/toggleSection() shell that nested object
sections already use, so the settings search's expand-on-match behavior
works on advanced fields with no JS changes. Object-type properties ignore
the flag (they already render as their own collapsible sections). No
backend change needed: jsonschema ignores unknown x-* keywords exactly as
it does for x-widget/x-propertyOrder.

Documented in docs/widget-guide.md alongside the other x-* extensions.

Validation (rendered with real Jinja, not just parsed):
- synthetic schema with 2 advanced fields: basic fields render before the
  section, advanced inside the collapsed shell, count badge correct,
  x-advanced on an object property correctly ignored
- schema without any x-advanced: output is identical to the pre-change
  template (whitespace-normalized diff against git HEAD's version)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

docs/widget-guide.md

@@ -206,6 +206,40 @@ To use an existing widget in your plugin's `config_schema.json`, simply add the
 
 The widget will be automatically rendered when the plugin configuration form is loaded.
 
+## Marking Fields as Advanced (`x-advanced`)
+
+Add `"x-advanced": true` to any top-level, non-object property to move it out
+of the main form and into a single collapsed **Advanced Settings** section at
+the bottom of the plugin's configuration page:
+
+```json
+{
+  "properties": {
+    "city": {
+      "type": "string",
+      "title": "City"
+    },
+    "request_timeout": {
+      "type": "integer",
+      "default": 10,
+      "description": "HTTP timeout in seconds",
+      "x-advanced": true
+    }
+  }
+}
+```
+
+Guidelines:
+
+- Use it for fine-tuning knobs most users never touch (timeouts, retry
+  behavior, cache TTLs, styling overrides). Anything a first-time user must
+  set to get the plugin working should stay basic.
+- Nothing is hidden permanently — the section expands on click, and the
+  settings search finds and auto-expands advanced fields like any others.
+- The flag is ignored on `object`-type properties (they already render as
+  their own collapsible sections) and is safely ignored by older cores, so
+  adding it never breaks compatibility.
+
 ## Creating Custom Widgets
 
 ### Step 1: Create Widget File