📝(docs) add side panel focus pattern

Document trigger ref and panel ref focus workflow

src/frontend/README.md

@@ -28,3 +28,36 @@ export default {
 - Replace `plugin:@typescript-eslint/recommended` to `plugin:@typescript-eslint/recommended-type-checked` or `plugin:@typescript-eslint/strict-type-checked`
 - Optionally add `plugin:@typescript-eslint/stylistic-type-checked`
 - Install [eslint-plugin-react](https://github.com/jsx-eslint/eslint-plugin-react) and add `plugin:react/recommended` & `plugin:react/jsx-runtime` to the `extends` list
+
+## Side Panel Focus Pattern
+
+We use a consistent focus management pattern for side panels:
+
+- **Open**: focus the first actionable element inside the panel.
+- **Close**: restore focus to the button that opened the panel.
+
+Implementation summary:
+
+1. A provider stores a `panelRef` and a registry of trigger refs (`setTrigger/getTrigger`).
+2. Each trigger button registers itself with `setTrigger("key", el)`.
+3. Panel content uses `useRestoreFocus` with:
+   - `resolveTrigger` → returns `getTrigger("key")`.
+   - `onOpened` → finds the first actionable element inside `panelRef`.
+
+Example:
+
+```tsx
+// Trigger button
+;<ToggleButton ref={(el) => setTrigger('tools', el)} />
+
+// Panel content
+useRestoreFocus(isOpen, {
+  resolveTrigger: (activeEl) => getTrigger('tools') ?? activeEl,
+  onOpened: () => {
+    const first = panelRef.current?.querySelector(
+      '[data-attr="tools-list"] button'
+    )
+    ;(first as HTMLElement | null)?.focus({ preventScroll: true })
+  },
+})
+```