fix(autocomplete): fix value persistence in DOM text widgets

Remove multiple sources of truth and async sync chains that caused
values to be lost during load/switch workflow or reload page.

Changes:
- Remove internalValue state variable from main.ts
- Update getValue/setValue to read/write DOM directly via widget.inputEl
- Remove textValue reactive ref and v-model from Vue component
- Remove serializeValue, onSetValue, and watch callbacks
- Register textarea reference on mount, clean up on unmount
- Simplify AutocompleteTextWidgetInterface

Follows ComfyUI built-in addMultilineWidget pattern:
- Single source of truth (DOM element value only)
- Direct sync (no intermediate variables or async chains)

Also adds documentation:
- docs/dom-widgets/value-persistence-best-practices.md
- docs/dom-widgets/README.md
- Update docs/dom_widget_dev_guide.md with reference

docs/dom_widget_dev_guide.md

@@ -240,6 +240,8 @@ inputEl.addEventListener("change", () => {
 });
 ```
 
+> **⚠️ Important**: For Vue-based DOM widgets with text inputs, follow the [Value Persistence Best Practices](dom-widgets/value-persistence-best-practices.md) to avoid sync issues. Key takeaway: use DOM element as single source of truth, avoid internal state variables and v-model.
+
 ### 5.3 The Restoration Mechanism (`configure`)
 
 *   **`configure(data)`**: When a Workflow is loaded, `LGraphNode` calls its `configure(data)` method.