Display a user-friendly error when connecting to a site that doesn't support HTTPS

## Context

From [PR #328 review](https://github.com/wordpress-mobile/GutenbergKit/pull/328#discussion_r2850038249):

The HTTP fallback in `SiteCapabilitiesDiscovery` doesn't work because the server isn't actually speaking HTTPS. When this happens, the user doesn't get a clear error message explaining the problem.

## Expected Behavior

When connecting to a site that doesn't support HTTPS, the app should display a clear, user-friendly error message explaining that the site doesn't support a secure connection.

## Current Behavior

The connection silently fails or falls through to default capabilities without informing the user about the HTTPS issue.

## Relevant Code

- `android/app/src/main/java/com/example/gutenbergkit/SiteCapabilitiesDiscovery.kt` — `discoverCapabilitiesViaHttp` method ([GutenbergKit PR #328](https://github.com/wordpress-mobile/GutenbergKit/pull/328))

Start in android/app/src/main/java/com/example/gutenbergkit/SiteCapabilitiesDiscovery.kt, focusing on discoverCapabilitiesViaHttp and the connection flow it uses. Reproduce or trace the fallback for a site that does not support HTTPS, then confirm that the app reports a clear, user-friendly secure-connection error instead of silently failing or returning default capabilities.

Written by the indexing model from the issue text.