feat: drop database from database switcher (#758)

[datlechin]

Closes #758

Summary

Adds the ability to drop/delete databases from the database switcher dialog.

Protocol + Capability

• dropDatabase(name:) added to PluginDatabaseDriver with default "not supported" implementation
• supportsDropDatabase capability flag in DriverPlugin and PluginMetadataRegistry
• No PluginKit version bump needed (additive default extension)

Plugin Implementations

• MySQL/MariaDB: DROP DATABASE
• PostgreSQL/Redshift: DROP DATABASE
• ClickHouse: DROP DATABASE
• SQL Server: DROP DATABASE
• MongoDB: runCommand dropDatabase
• Cassandra: DROP KEYSPACE
• BigQuery: DROP SCHEMA
• Cloudflare D1: HTTP DELETE API

UI

• Trash button in database switcher toolbar
• Right-click context menu "Drop Database..." on database rows
• Delete key shortcut when database selected
• Confirmation dialog with warning, loading state, error display
• Disabled for system databases, currently connected database, and schema mode

Test plan

• Build passes
• MySQL: right-click non-system database, "Drop Database...", confirm, verify deleted
• PostgreSQL: verify error when trying to drop currently connected database
• System databases (information_schema, mysql, postgres) have drop disabled
• Currently connected database has drop disabled
• Delete key triggers confirmation
• Schema mode hides drop button
• SQLite/Redis: no drop button shown