feat(12-02): make config sections collapsible with chevron dropdown

Sections start collapsed showing only heading, description, and a
chevron icon. Click to expand and reveal the fields. The open property
allows sections to start expanded if needed.

frontend/src/components/config-page/config-section.ts

@@ -1,9 +1,10 @@
 import { LitElement, html, css, nothing } from 'lit';
-import { customElement, property } from 'lit/decorators.js';
+import { customElement, property, state } from 'lit/decorators.js';
 
 /**
  * A visual grouping wrapper for config fields.
- * Renders a heading, optional description, and a slot for fields.
+ * Renders a collapsible heading with optional description,
+ * and a slot for fields. Starts collapsed by default.
  */
 @customElement('config-section')
 export class ConfigSection extends LitElement {
@@ -17,11 +18,42 @@ export class ConfigSection extends LitElement {
             background: var(--yj-bg-surface, #212529);
             border: 1px solid var(--yj-border-subtle, #333);
             border-radius: 6px;
+        }
+
+        .header {
+            display: flex;
+            align-items: flex-start;
+            gap: 0.5em;
             padding: 1.25em;
+            cursor: pointer;
+            user-select: none;
+        }
+
+        .header:hover {
+            background: var(--yj-bg-elevated, #343a40);
+            border-radius: 6px;
+        }
+
+        .chevron {
+            flex-shrink: 0;
+            width: 16px;
+            height: 16px;
+            margin-top: 0.15em;
+            transition: transform 150ms ease;
+            color: var(--yj-text-tertiary, #888);
+        }
+
+        .chevron.open {
+            transform: rotate(90deg);
+        }
+
+        .header-text {
+            flex: 1;
+            min-width: 0;
         }
 
         h3 {
-            margin: 0 0 0.25em;
+            margin: 0;
             font-size: 1em;
             font-weight: 700;
             color: var(--yj-text-primary, #fff);
@@ -30,7 +62,11 @@ export class ConfigSection extends LitElement {
         .description {
             font-size: 0.8em;
             color: var(--yj-text-tertiary, #888);
-            margin: 0 0 1em;
+            margin: 0.25em 0 0;
+        }
+
+        .body {
+            padding: 0 1.25em 1.25em;
         }
 
         .fields {
@@ -45,18 +81,55 @@ export class ConfigSection extends LitElement {
     @property({ type: String })
     description = '';
 
+    @property({ type: Boolean })
+    open = false;
+
+    @state()
+    private expanded = false;
+
+    override connectedCallback(): void {
+        super.connectedCallback();
+        this.expanded = this.open;
+    }
+
+    private toggle = (): void => {
+        this.expanded = !this.expanded;
+    };
+
     override render() {
         return html`
             <div class="section">
-                <h3>${this.heading}</h3>
-                ${this.description
-                    ? html`<p class="description">
-                          ${this.description}
-                      </p>`
-                    : nothing}
-                <div class="fields">
-                    <slot></slot>
+                <div
+                    class="header"
+                    @click=${this.toggle}
+                >
+                    <svg
+                        class="chevron ${this.expanded ? 'open' : ''}"
+                        viewBox="0 0 16 16"
+                        fill="currentColor"
+                    >
+                        <path
+                            d="M6 3l5 5-5 5z"
+                        />
+                    </svg>
+                    <div class="header-text">
+                        <h3>${this.heading}</h3>
+                        ${this.description
+                            ? html`<p class="description">
+                                  ${this.description}
+                              </p>`
+                            : nothing}
+                    </div>
                 </div>
+                ${this.expanded
+                    ? html`
+                          <div class="body">
+                              <div class="fields">
+                                  <slot></slot>
+                              </div>
+                          </div>
+                      `
+                    : nothing}
             </div>
         `;
     }