Auto-load graph-explorer-config.json on startup

## Description

When deploying Graph Explorer via Docker, users can export their configuration (connections, styles, schemas, etc.) using Settings → General → Save, which creates a `graph-explorer-config.json` file. Currently, this file must be manually loaded via the UI each time, even when mounted as a volume in Docker.

While `defaultConnection.json` is automatically loaded for connection settings, the full backup configuration file (`graph-explorer-config.json`) containing styles, schemas, and other customizations is not automatically loaded on startup.

I'm frustrated when I have to manually click "Load" in the Settings UI every time I access Graph Explorer, even though I've mounted the config file as a Docker volume. This makes it difficult to use infrastructure-as-code approaches and share configurations across team members.

## Preferred Solution

Implement automatic loading of `graph-explorer-config.json` on application startup, similar to how `defaultConnection.json` is currently auto-loaded.

**Proposed Implementation:**

1. **Proxy Server**: Serve `graph-explorer-config.json` from `CONFIGURATION_FOLDER_PATH` (similar to `defaultConnection.json`)
- Add route: `GET /graph-explorer-config.json`
- Serve file from: `${CONFIGURATION_FOLDER_PATH}/graph-explorer-config.json`

2. **Frontend Auto-Load**: Before React initializes (in `storageAtoms.ts`), check if:
- No existing configuration exists in IndexedDB
- Backup config file is available at `/graph-explorer-config.json`
- If both conditions are met, automatically fetch and restore the backup using the existing `restoreBackup()` function

3. **Safety**: Only auto-load if IndexedDB is empty (no existing user data) to avoid overwriting user configurations

**Expected Behavior:**
- `defaultConnection.json` → ✅ Auto-loads (connections only) - already works
- `graph-explorer-config.json` → ✅ Auto-loads (full config) when IndexedDB is empty - new behavior

## Additional Context

**Current Behavior:**
- `defaultConnection.json` → ✅ Auto-loads (connections only)
- `graph-explorer-config.json` → ❌ Manual load required (full config with styles, schemas, etc.)

**Docker Compose Example:**

```docker
services:
graph-explorer:
volumes:
- ./config/graph-explorer:/graph-explorer-config
environment:
- CONFIGURATION_FOLDER_PATH=/graph-explorer-config
```

Start by reading storageAtoms.ts, the existing defaultConnection.json startup loading path, and the restoreBackup() function. Then inspect the proxy server route that serves defaultConnection.json and add the parallel configuration-file behavior described in the issue. Done means a mounted graph-explorer-config.json is fetched and restored only when IndexedDB has no existing configuration, without overwriting user data.

Written by the indexing model from the issue text.