feat: show all databases in sidebar tree

[datlechin]

Summary

Add a "Show all databases" feature to the sidebar, displaying all databases on the server as a tree structure (similar to DBeaver).

Currently, the sidebar only shows tables for the connected database. Users (especially testers) need to access multiple databases at once to check permissions — if not authorized, it shows a permission error; if authorized, they can access and work with the data.

Motivation

From community feedback: testers need to quickly switch between databases to verify access permissions. The existing Cmd+K shortcut only opens schemas within the current database, not other databases on the server.

Proposed Approach

Replace the flat table list in the sidebar with a hierarchical database tree:

▼ my_app_db (current)
    users
    orders
    products
▸ analytics_db
▸ test_db
▸ staging_db

• The current database is auto-expanded showing its tables
• Other databases show as collapsed nodes
• Expanding a database lazy-loads its tables (via existing fetchDatabases() / switchDatabase() infrastructure)
• Clicking a database switches the active context

Implementation Notes

• All drivers already implement fetchDatabases() (SHOW DATABASES for MySQL, pg_database query for PostgreSQL, listDatabases() for MongoDB)
• switchDatabase(to:) in MainContentCoordinator+Navigation.swift handles the context switch
• PostgreSQL is special: should show schemas (switchable via SET search_path) rather than databases (which require full reconnection)
• SQLite is file-based, so this feature would be a no-op for SQLite connections

Reference

DBeaver's "Show all databases" UI: