feat: when restored position has no nodes in viewport, automatically fit to view (#7435)

## Summary

Sometimes the saved position is super far away from any of the nodes,
which causes general confusion. This PR changes the `loadGraphData`
logic to fit-to-view in those scenarios.

Fixes https://github.com/Comfy-Org/ComfyUI_frontend/issues/7425

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7435-feat-when-restored-position-has-no-nodes-in-viewport-automatically-fit-to-view-2c86d73d36508119bf2ed9d361ec868f)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

browser_tests/tests/viewport.spec.ts

@@ -0,0 +1,20 @@
+import { expect } from '@playwright/test'
+
+import { comfyPageFixture as test } from '../fixtures/ComfyPage'
+
+test.describe('Viewport', () => {
+  test('Fits view to nodes when saved viewport position is offscreen', async ({
+    comfyPage
+  }) => {
+    await comfyPage.loadWorkflow('viewport/default-viewport-saved-offscreen')
+
+    // Wait a few frames for rendering to stabilize
+    for (let i = 0; i < 5; i++) {
+      await comfyPage.nextFrame()
+    }
+
+    await expect(comfyPage.canvas).toHaveScreenshot(
+      'viewport-fits-when-saved-offscreen.png'
+    )
+  })
+})