Warn users with a banner when App Proxy or Storage Proxy is unreachable

Users hitting a blocked/firewalled network see cryptic failures (e.g. `AppLaunchError: Proxy configurator is not responding`) only at the moment they try to launch an app, with no earlier indication that connectivity is broken. Support cases like this are hard to diagnose and are often misattributed to transient network issues.

WebUI should proactively probe the **App Proxy coordinator URL** and the **Storage Proxy client-facing URL** — periodically, and/or when entering a page that depends on them — and surface a warning banner when a probe fails or times out, so users notice the connectivity problem before it manifests as an app-launch error.

**Requirements**

- Add two separate Alert banner components: one for App Proxy, one for Storage Proxy.
- Suppress both service-specific banners while the existing full-offline banner is showing (total network loss takes precedence; do not stack redundant warnings).
- Message wording: state that the connection to the given proxy is not working, and prompt the user to check their firewall or network status — e.g. "Connection to the OOO proxy is unstable. Please check your firewall or network status." (exact copy TBD; must be i18n-ready).
- No dedicated page — users will not navigate to a diagnostics page on their own. The banner must appear in-context on the pages where it matters.

**Notes**

- Related to the existing offline-banner probe work in FR-3237.
- A real support case traced this class of error to a firewall blocking the proxy endpoint; the user had no way to know that from the UI.

JIRA Issue: FR-3477

No files or tests are named; start by reading the existing offline-banner probe work in FR-3237 and locating the WebUI pages that depend on the App Proxy coordinator and Storage Proxy client-facing URLs. Done means separate i18n-ready banners probe both URLs, are suppressed by the full-offline banner, and communicate firewall or network checks on failure or timeout.

Written by the indexing model from the issue text.