feat: dynamic container entity name + BigQuery dataset switching UX

[datlechin]

Summary

• Add containerEntityName to DriverPlugin protocol — each database declares its container name (BigQuery → "Dataset", Cassandra → "Keyspace", others → "Database")
• Enable ⌘K (Open Database) menu for schema-switching databases like BigQuery
• Hide Databases/Schemas segmented picker when only schema switching is supported
• Replace 7 hardcoded "Database" labels with dynamic containerEntityName lookup

Changes

• DriverPlugin.swift: new containerEntityName property with "Database" default
• PluginMetadataRegistry.swift: add to SchemaInfo, defaults, snapshot builder
• PluginManager.swift: add containerEntityName(for:) accessor
• BigQueryPlugin.swift: containerEntityName = "Dataset"
• ContentView.swift: enable ⌘K for supportsSchemaSwitching databases
• TableProApp.swift: dynamic menu label
• TableProToolbarView.swift: dynamic toolbar label + tooltip
• MainEditorContentView.swift: dynamic empty state
• DatabaseSwitcherSheet.swift: dynamic header, hide picker for schema-only DBs
• MainContentCoordinator+Navigation.swift: dynamic error title
• SidebarView.swift: dynamic empty state

Test plan

• BigQuery: menu shows "Open Dataset...", toolbar "Dataset", empty state "Switch Dataset"
• MySQL/PostgreSQL: all show "Database" (unchanged)
• Cassandra: shows "Keyspace"
• ⌘K works for BigQuery (schema-switching only)
• No crash with existing plugins (default "Database")