Expose CLI download settings (cliDownloadUrl / allowCLIDownloadPanel) via appConfig

## Background

WebUI FR-3226 adds a "Download CLI" surface controlled by two settings:

- `cliDownloadUrl` — base URL for the CLI SCIE binaries (defaults to `https://github.com/lablup/backend.ai/releases/latest/download`).
- `allowCLIDownloadPanel` — whether the CLI download tab is shown (defaults to `true`).

The WebUI reads these from the webserver-provided `config.toml` when present, but **the webserver does not ship them in its config yet**, so today they only resolve from the frontend defaults. `config.toml.sample` documents them on the WebUI side as a stopgap.

## Scope note

**This is a backend / appConfig task** (belongs in the BA project — could not be created there via the jira CLI because BA requires a Story point estimate the tool can't set). Filed here as a tracker linked to FR-3226; please move to BA.

## Task

Expose `cliDownloadUrl` / `allowCLIDownloadPanel` through **appConfig** so they can be centrally configured and served to the WebUI, instead of living only as frontend defaults + local config.toml.

## Reference

- Frontend: `react/src/helper/loginConfig.ts` (defaults + parsing), `config.toml.sample`.
- Related: FR-3226 (WebUI CLI download feature).

JIRA Issue: FR-3236

Start with react/src/helper/loginConfig.ts and config.toml.sample to understand the existing defaults and parsing for cliDownloadUrl and allowCLIDownloadPanel. Then trace the appConfig path that serves WebUI configuration; done means both settings can be centrally configured and are delivered to the WebUI instead of relying only on frontend defaults.

Written by the indexing model from the issue text.