feat(connections): edit existing connection

[ZhuchkaTriplesix]

Summary

• Context menu Edit connection… on PG/MySQL/SQLite/Redis/Mongo/extension/fallback tiles
• Forms accept initial: — type fixed, password left blank (“keep existing”), URI passwords redacted
• mergeSecretsForConnectionUpdate before LocalDb.updateConnection; reconnect + refresh workspace when active/expanded

Closes #510

Test plan

• Right-click a saved PG connection → Edit → change host → Save → sidebar updates
• Leave password blank → old password still works on connect
• Replace password → new password used after reconnect
• Cancel leaves connection unchanged
• Active expanded connection reconnects after credential/host change
• Same for MySQL / Redis / Mongo / SQLite / extension driver
• flutter test test/features/connections/connection_edit_helpers_test.dart test/core/storage/local_db_secrets_test.dart test/features/postgresql/postgresql_connection_form_test.dart