feat(python-notebook-migration, frontend): add Jupyter notebook iframe panel component

[zyratlo]

What changes were proposed in this PR?

Introduces JupyterNotebookPanelComponent, the draggable iframe shell that hosts JupyterLab inside the Texera workspace. It consumes the visibility surface added in migration-tool-panel-controls (jupyterNotebookPanelVisible$ / closeJupyterNotebookPanel / minimizeJupyterNotebookPanel) and registers itself with the panel service via setIframeRef once its ViewChild is in the DOM.

• frontend/src/app/workspace/component/jupyter-notebook-panel/jupyter-notebook-panel.component.ts
  • JupyterNotebookPanelComponent
    • ngOnInit subscribes to jupyterPanelService.jupyterNotebookPanelVisible$; on each emission it sets isVisible, and when visibility flips to true it fetches the iframe URL from notebookMigrationService.getJupyterIframeURL(), runs it through DomSanitizer.bypassSecurityTrustResourceUrl, and updates iframeRef
    • ngAfterViewInit registers the iframe with the panel service via setIframeRef
    • closePanel() calls jupyterPanelService.closeJupyterNotebookPanel()
    • minimizePanel() calls jupyterPanelService.minimizeJupyterNotebookPanel().
• frontend/src/app/workspace/component/jupyter-notebook-panel/jupyter-notebook-panel.component.html
  • cdkDrag-enabled panel with header (title + close/minimize buttons; close uses nz-popconfirm for a "delete" confirmation)
  • <iframe [src]="jupyterUrl"> body bound to the sanitized URL, gated on *ngIf="isVisible".
• frontend/src/app/workspace/component/jupyter-notebook-panel/jupyter-notebook-panel.component.scss
  • fixed-position panel styling, drag handle visuals, iframe sizing.
• frontend/src/app/workspace/component/jupyter-notebook-panel/jupyter-notebook-panel.component.spec.ts
• frontend/src/app/app.module.ts
  • imports JupyterNotebookPanelComponent and adds it to the @NgModule.declarations array.

Any related issues, documentation, discussions?

Closes #5270
Parent issue #4301

How was this PR tested?

jupyter-notebook-panel.component.spec.ts

• mocks JupyterPanelService (via a controllable Subject<boolean> for the visibility observable) and NotebookMigrationService (returning a fixture URL)
• Covers visibility transitions, URL fetch + sanitization, ref registration, button delegation.

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