fix: persist SSH jumphost and tunnel mode in connections.json

[datlechin]

Summary

Closes #790. Partial fix for #780.

Root cause of #790: StoredConnection used flat scalar fields for SSH config (sshHost, sshPort, etc.) but never persisted jumpHosts or sshTunnelMode. Every save/load cycle silently dropped all jump host configuration. First connect worked (in-memory object correct), but reconnecting after app restart failed because connections.json had no jumphost data.

Changes

StoredConnection — sshTunnelModeJson field:

• Added sshTunnelModeJson: Data? that encodes the full SSHTunnelMode enum (already Codable, includes inline config with jumpHosts, profile with snapshot)
• On load: prefers JSON blob when present, falls back to legacy flat fields for old connections.json files
• On save: writes both JSON blob (source of truth) and flat fields (backward compat with older app versions)
• No migration needed. Old files decode via decodeIfPresent returning nil, legacy flat-field path runs unchanged. Next save writes the blob.

KeychainHelper — loadWithStatus:

• Added KeychainLoadResult enum: .success(Data), .notFound, .locked, .error(OSStatus)
• Added loadWithStatus(key:) that distinguishes errSecInteractionNotAllowed (Keychain locked before first unlock after reboot) from errSecItemNotFound
• Existing load(key:) -> Data? preserved for backward compat

duplicateConnection — pass sshTunnelMode:

• Now passes sshTunnelMode: connection.sshTunnelMode explicitly instead of relying on auto-derive logic

Export — sshProfileId:

• Added sshProfileId: String? to ExportableConnection
• Export includes profile binding; import restores it

Test plan

• Create MySQL connection with SSH tunnel + jumphost, save, quit app, reopen - jumphost still present
• Connect with jumphost - succeeds
• Duplicate connection with jumphost - duplicate has jumphost
• Old connections.json without sshTunnelModeJson key loads correctly (flat fields used)
• Export/import connection with SSH profile - profile binding preserved