perf(storage): resolve connection secrets lazily and handle Keychain errors gracefully (#916)

[ZhuchkaTriplesix]

Summary

• Avoid unbounded parallel OS Keychain / D-Bus secret reads on app startup by making LocalDb.getConnections() lazy by default (hydrateSecrets: false), as the sidebar connection tree only requires basic metadata.
• Wrap ConnectionSecretsStore.readForConnection and LocalDb._hydrateConnection in try/catch to gracefully swallow platform Keychain / D-Bus errors without failing connection tree rendering.
• Expose on-demand hydration helpers (LocalDb.hydrateConnection and getConnectionById) while keeping driver on-connect secret resolution intact.
• Update tests and test memory backend with failNextRead to verify graceful degradation and lazy secret resolution.

Linked issue

Closes #916

Checklist

• flutter analyze passes locally
• Scope matches the linked issue only (no drive-by refactors)
• PR targets dev