Persistence migration for saved connections

[kmcginnes]

Parent PRD

• Parent Simplify client-server relationship by removing proxy URL from connection config #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 Simplify ConnectionConfig — remove url and proxyConnection #1622

User stories addressed

• User story 6