Remove the proxy server URL from connections — make it automatic

## Desired outcome

A user should never have to think about a **proxy server URL** — not per connection, and not at all. Today, creating a connection forces the user to specify a "Public or Proxy Endpoint" URL and toggle a "Using Proxy-Server" checkbox, even though the client is always served by that same proxy and already knows its own origin. This is redundant, error-prone, and confusing.

Instead, the client should **automatically** route its requests through the proxy that serves it, resolving the endpoint from its own location. The connection form should collapse to just the **graph database URL** (plus optional auth settings). Existing saved connections that still carry the old proxy fields should migrate transparently, with no user action.

## Acceptance criteria

- [ ] Creating or editing a connection requires only the graph database URL (and optional auth) — no proxy endpoint field, no proxy-server toggle.
- [ ] Client requests reach the database through the serving proxy with no per-connection URL configuration, behind any reverse proxy or deployment path.
- [ ] Previously saved connections continue to work, migrated automatically on load.

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.