refactor: create and extend browser test helpers for ComfyPage extraction

Phase 1 of ComfyPage refactor - extract method groups into helpers:

- New ToastHelper: toast error/visibility/close methods

- New DragDropHelper: file and URL drag-and-drop methods

- New CommandHelper: command execution and registration

- Extended WorkflowHelper: undo/redo queue, modified state, export

- Extended CanvasHelper: group position/drag, edge connect/disconnect

- Extended NodeOperationsHelper: prompt dialog, node drag, widget adjust

Amp-Thread-ID: https://ampcode.com/threads/T-019c165a-5aba-7582-9f2b-e5b2bba2783b
Co-authored-by: Amp <amp@ampcode.com>

browser_tests/fixtures/helpers/CommandHelper.ts

@@ -0,0 +1,68 @@
+import type { Page } from '@playwright/test'
+
+import type { KeyCombo } from '../../../src/platform/keybindings'
+
+export class CommandHelper {
+  constructor(private readonly page: Page) {}
+
+  async executeCommand(commandId: string): Promise<void> {
+    await this.page.evaluate((id: string) => {
+      return window['app'].extensionManager.command.execute(id)
+    }, commandId)
+  }
+
+  async registerCommand(
+    commandId: string,
+    command: (() => void) | (() => Promise<void>)
+  ): Promise<void> {
+    await this.page.evaluate(
+      ({ commandId, commandStr }) => {
+        const app = window['app']
+        const randomSuffix = Math.random().toString(36).substring(2, 8)
+        const extensionName = `TestExtension_${randomSuffix}`
+
+        app.registerExtension({
+          name: extensionName,
+          commands: [
+            {
+              id: commandId,
+              function: eval(commandStr)
+            }
+          ]
+        })
+      },
+      { commandId, commandStr: command.toString() }
+    )
+  }
+
+  async registerKeybinding(
+    keyCombo: KeyCombo,
+    command: () => void
+  ): Promise<void> {
+    await this.page.evaluate(
+      ({ keyCombo, commandStr }) => {
+        const app = window['app']
+        const randomSuffix = Math.random().toString(36).substring(2, 8)
+        const extensionName = `TestExtension_${randomSuffix}`
+        const commandId = `TestCommand_${randomSuffix}`
+
+        app.registerExtension({
+          name: extensionName,
+          keybindings: [
+            {
+              combo: keyCombo,
+              commandId: commandId
+            }
+          ],
+          commands: [
+            {
+              id: commandId,
+              function: eval(commandStr)
+            }
+          ]
+        })
+      },
+      { keyCombo, commandStr: command.toString() }
+    )
+  }
+}