Simplify CreateConnection form UI

## Parent PRD

- Parent #1618

## What to build

Update the `CreateConnection` form to remove proxy-related fields. Remove the "Public or Proxy Endpoint" text area, remove the "Using Proxy-Server" checkbox, and promote `graphDbUrl` ("Graph Connection URL") as the primary and only URL input field.

**Type: HITL** — requires review before completion.

## Acceptance criteria

- [ ] "Public or Proxy Endpoint" field is removed from the form
- [ ] "Using Proxy-Server" checkbox is removed from the form
- [ ] `graphDbUrl` is the primary URL field, always visible, labeled clearly (e.g., "Graph Database URL")
- [ ] Form validation requires `graphDbUrl`
- [ ] Submitting the form produces a valid `ConnectionConfig` with `graphDbUrl`
- [ ] Tests verify the form renders without proxy-related fields
- [ ] Tests verify form submission produces correct data shape

## Blocked by

- Blocked by #1622

## User stories addressed

- User story 1
- User story 2