[Feat] [Dashboard/Frontend] Add configurable port support for Open WebUI iframe

[samzong]

Use iframe with configurable port for Open WebUI integration

Why

In all-in-one container deployments, Open WebUI doesn't support subpath-based access. Using a reverse proxy approach creates numerous conflict issues. This PR switches to an iframe-based integration with direct port access, which for a single Docker container simply means exposing an additional port.

What's Changes

• Add VITE_OPENWEBUI_PORT environment variable support for configuring Open WebUI port
• Update iframe URL logic to prioritize configured port over hostname pattern matching
• Improve error handling when Open WebUI URL cannot be determined
• Update user-facing message to reflect new configuration approach

Special Note

This simplifies deployment in containerized environments while maintaining flexibility for other deployment scenarios.

FIX #xxxx (link existing issues this PR will resolve)

BEFORE SUBMITTING, PLEASE READ THE CHECKLIST BELOW AND FILL IN THE DESCRIPTION ABOVE

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 dashboard

Owners: @JaredforReal, @Xunzhuo
Files changed:

• dashboard/frontend/src/pages/PlaygroundPage.tsx
• dashboard/frontend/src/vite-env.d.ts

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🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[rootfs]

@JaredforReal PTAL, thanks

[JaredforReal]

@rootfs, we are waiting for #845

[rootfs]

@JaredforReal thanks for the detail, let's fast track #845