Persistence migration for saved connections

## Parent PRD

- Parent #1618

## What to build

Add a migration that runs on app startup when loading connection data from IndexedDB. Detect old `ConnectionConfig` shapes that contain `url` or `proxyConnection` fields and migrate them to the new shape: map `graphDbUrl || url` → `graphDbUrl`, drop `url` and `proxyConnection`.

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

## Acceptance criteria

- [ ] Old connections with `graphDbUrl` set are migrated (keep `graphDbUrl`, drop `url` and `proxyConnection`)
- [ ] Old connections with only `url` set (no `graphDbUrl`) fall back to using `url` as `graphDbUrl`
- [ ] Already-migrated data passes through unchanged
- [ ] Migration runs automatically on app startup
- [ ] Tests cover all migration scenarios: both fields present, only `url`, only `graphDbUrl`, already migrated

## Blocked by

- Blocked by #1622

## User stories addressed

- User story 6

Start by locating the app-startup path that loads connection data from IndexedDB and the ConnectionConfig definition. Add coverage for both fields, only url, only graphDbUrl, and already-migrated data, then verify the migration runs automatically during startup; issue #1622 must be resolved first and completion requires review.

Written by the indexing model from the issue text.