Improve schema sync performance for large schemas

## Description

Schema sync fires thousands of sequential network requests when discovering a schema with 70k+ node types. Gremlin batches 100 types per request (700 batches), while openCypher and SPARQL fire one request per type at concurrency 10 (7,000 sequential batches). This makes initial schema discovery take an extremely long time with no user feedback or cancellation option.

## Preferred Solution

### 1. Add progress reporting and cancellation to schema sync

Show a progress indicator during schema discovery so users know how far along the process is and can cancel if needed. The current UI gives no feedback during the potentially long sync.

### 2. Increase batch concurrency

`DEFAULT_CONCURRENT_REQUESTS_LIMIT` is 10 and `DEFAULT_BATCH_REQUEST_SIZE` is 100. Increase these values, especially for openCypher and SPARQL which currently fire one request per type. Consider batching multiple types into a single query where the query language supports it.

### 3. Consider lazy attribute fetching

Instead of fetching attribute details for all 70k types upfront, only fetch attributes for types the user actually interacts with (expands in the explorer, selects in data explorer, etc.). Store a lightweight type list initially and fetch full type configs on demand.

### 4. Optimize IndexedDB persistence for large schemas

With 70k types, the serialized schema can be ~43-86 MB. Deserialization on app startup blocks rendering. Consider chunked persistence or lazy loading of the schema from IndexedDB.

## Related Issues

- Part of #1677

Start by locating the schema discovery and sync code, the DEFAULT_CONCURRENT_REQUESTS_LIMIT and DEFAULT_BATCH_REQUEST_SIZE constants, the schema-sync UI, and IndexedDB persistence. Compare the openCypher and SPARQL request paths with Gremlin batching, then define a focused implementation plan whose result reports progress, supports cancellation, reduces requests, and avoids blocking startup for very large schemas.

Written by the indexing model from the issue text.