Bug: Voice mode cannot be enabled - Failed to fetch model catalog (catalog unreachable on corporate VPN)

## Bug: Cannot enable voice mode — "Failed to fetch model catalog" / "Couldn''t load voice model catalog"

### Summary
Activating voice mode with `/voice` fails because the CLI cannot fetch the voice (STT) model catalog. Voice mode can''t be enabled at all, since the catalog must be reachable before any speech-to-text model can be downloaded.

### What I see
Two dialogs appear when trying to enable voice mode:

**1) Voice models picker:**
```
Voice models

No STT models available.
Your CLI may be out of date. Run /update to update, or download from:
https://github.com/github/copilot-cli/releases

Failed to fetch model catalog. Press r to retry.
```

**2) Blocking dialog:**
```
Couldn''t load voice model catalog
The voice model catalog couldn''t be fetched.

Voice mode can''t be enabled until the catalog is reachable. Check your network connection and retry.

> Retry
Cancel
```

Pressing `r` / **Retry** does not resolve it — the catalog still fails to load and no STT models become available, so voice mode never enables.

### Steps to reproduce
1. Run the Copilot CLI.
2. Invoke `/voice` to enable voice mode (dictation transcription via Foundry Local).
3. The CLI attempts to fetch the voice model catalog and fails with the errors above.

### Environment
- OS: Windows 11 (arm64)
- Copilot CLI version: 1.0.57
- Network: corporate environment. The machine was connected to a corporate VPN (Azure VPN). The failure occurs in this restricted-network context, which strongly suggests the model-catalog / model-download endpoints are being blocked by the VPN/proxy/firewall.

### Expected behavior
- Voice mode should be able to fetch the model catalog and download STT models in corporate/proxy/VPN environments, OR
- The error should clearly identify the exact endpoint(s) it''s trying to reach (host/URL) and respect system/HTTP(S) proxy settings, so the URLs can be allow-listed.

### Impact
Voice mode is completely unusable in restricted-network/VPN environments because the catalog fetch is a hard prerequisite and there is no offline / manual model-install fallback surfaced in the UI.

### Suggested improvements
- Surface the exact catalog/model URLs in the error so they can be allow-listed.
- Honor `HTTPS_PROXY` / system proxy and custom CA settings for the catalog + model download.
- Provide an offline / manual model-install path when the catalog is unreachable.

### Screenshots

Bắt đầu với flow /voice và tái hiện trên Windows 11 arm64 trong khi kết nối với Azure VPN của công ty. Theo dõi request đến danh mục model giọng nói của Foundry Local và đường dẫn tải model, đồng thời ghi lại endpoint và cách proxy hoặc custom-CA hoạt động. Được xem là hoàn tất khi voice mode có thể tải các model STT trong mạng bị hạn chế đã báo cáo hoặc UI hiển thị hướng dẫn có thể thực hiện về endpoint và proxy.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.