fix: add isGraphReady guard to prevent premature graph access error logs (#9672)

## Summary
Adds `isGraphReady` getter to `ComfyApp` and uses it in
`executionErrorStore` guards to prevent false 'ComfyApp graph accessed
before initialization' error logs during early store evaluation.

## Changes
- **What**: Added `isGraphReady` boolean getter to `ComfyApp` that
safely checks graph initialization without triggering the `rootGraph`
getter's error log. Updated 5 guard sites in `executionErrorStore` to
use `app.isGraphReady` instead of `app.rootGraph`.
- **Why**: The `rootGraph` getter logs an error when accessed before
initialization. Computed properties and watch callbacks in
`executionErrorStore` are evaluated early (before graph init), causing
false error noise in the console.

## Review Focus
- `isGraphReady` is intentionally minimal — just
`!!this.rootGraphInternal` — to avoid duplicating the error-logging
behavior of `rootGraph`
- The `watch(lastNodeErrors, ...)` callback now checks `isGraphReady` at
the top and early-returns, consistent with the computed property pattern

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9672-fix-add-isGraphReady-guard-to-prevent-premature-graph-access-error-logs-31e6d73d365081be8e1fc77114ce9382)
by [Unito](https://www.unito.io)

Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/scripts/app.ts

@@ -208,6 +208,11 @@ export class ComfyApp {
     return this.rootGraphInternal!
   }
 
+  /** Whether the root graph has been initialized. Safe to check without triggering error logs. */
+  get isGraphReady(): boolean {
+    return !!this.rootGraphInternal
+  }
+
   canvas!: LGraphCanvas
   dragOverNode: LGraphNode | null = null
   readonly canvasElRef = shallowRef<HTMLCanvasElement>()