fix: rewrite database/schema switching — persist schema, fix reconnect, crash-safe session list (#777)

[datlechin]

Closes #777

Summary

Full rewrite of the database/schema switching system. Fixes 8 bugs across persistence, encapsulation, and crash safety.

Fixes

• Schema not restored on reconnect for PostgreSQL/Redshift/BigQuery (Persist last-used schema per connection #777)
• Database not restored on reconnect for MySQL/MariaDB/ClickHouse (missing postConnectActions)
• Database lost after auto-reconnect (clearCachedData was niling currentDatabase)
• selectDatabaseFromConnectionField set state before error check (Redis state corruption)
• Database switch closed windows before driver call confirmed success (UI damaged on failure)
• Schema-switch branch saved to wrong UserDefaults key (lastDatabase instead of lastSchema)
• Schema switch didn't save tabs before clearing (tab state lost)
• Open connection IDs only saved in applicationWillTerminate (Cmd+R in Xcode lost current connection)
• Orphaned UserDefaults keys on connection deletion

Architecture

• DatabaseManager.switchDatabase(to:for:) and switchSchema(to:for:) own driver call + session state + persistence
• Coordinator only handles UI effects after manager confirms success
• PostConnectAction.selectSchemaFromLastSession for PostgreSQL/Redshift/BigQuery
• All plugins declare their postConnectActions explicitly (not relying on registry defaults)
• Open connection IDs persisted on every session add/remove via activeSessions.didSet

Test plan

• Build passes
• PostgreSQL: switch schema, Cmd+R, reconnect - schema restored
• MySQL/MariaDB: empty DB field, switch database, reconnect - database restored
• ClickHouse: switch database, reconnect - database restored
• MSSQL: auto-reconnect after drop - database restored
• Switch to non-existent database - windows and tabs preserved, error shown
• Delete connection - no orphaned UserDefaults keys
• Cmd+R in Xcode - correct connection restored on relaunch