[V2 UI] Missing model links trigger local browser download instead of server-side fetch on remote/headless hosts

### Feature Idea

Subject: [V2 UI] Missing Model links trigger local browser download instead of server-side download
Description:
On the new ComfyUI V2 interface (April 2026 build), when a workflow has missing models, the UI provides a direct download link. When accessing the UI from a mobile browser (Brave on Android) connected to a remote headless server (Ubuntu 24.04), clicking these links triggers a local download to the mobile device instead of instructing the server to fetch the model.
Environment:
Server: MSI EdgeXpert (NVIDIA Blackwell), Ubuntu 24.04 LTS (Headless/TTY mode).
Client: Brave Browser on Android (Mobile).
Connection: Local LAN / Tailscale.
ComfyUI Version: V2 (New UI).
Steps to Reproduce:
Load a workflow with a missing model (e.g., z_image_bf16.safetensors).
Click the provided download link/popup in the V2 UI.
The mobile browser attempts to save the ~11GB file to the phone storage rather than the server's models/checkpoints directory.
Expected Behavior:
The UI should provide an option to trigger a server-side fetch (similar to ComfyUI-Manager).

### Existing Solutions

Manual search and download.

### Other

_No response_

Start by reproducing the missing-model download flow in the V2 UI from a remote headless server, then trace the existing download link or popup entry point. Check how ComfyUI-Manager handles server-side model fetching and identify the relevant UI and server components. Done means the interface offers a server-side fetch option without forcing the model download onto the client, with coverage for the remote-browser case.

Written by the indexing model from the issue text.