fix: crash when creating database on PostgreSQL (macOS 26)

[datlechin]

Summary

• Fix EXC_BAD_ACCESS crash in swift_retain when clicking Create in the Create Database dialog on PostgreSQL connections running macOS 26 (Tahoe)
• Root cause: macOS 26 Swift runtime injects Builtin.ImplicitActor into async closure thunks, shifting String argument slots and corrupting them with the MainActor protocol witness table address
• Fix: pass DatabaseSwitcherViewModel directly to CreateDatabaseSheet instead of an async onCreate closure, eliminating the problematic thunk chain entirely

Test plan

• Connect to PostgreSQL, open Create Database dialog, enter a name, click Create — no crash
• Verify the database is actually created on the server
• Verify the database list refreshes after creation
• Test Cancel button still works
• Test on macOS 15 (Sequoia) to confirm no regression