Expand GraphQL explorer query and variable builder

[xaaha]

• add a dedicated query preview panel and variable preview panel in the GraphQL explorer so users can see the generated operation and variables alongside the form
• build GraphQL query strings from the current form state, including enabled arguments, nested field selections, inline fragments, unions, and interfaces
• add variable input handling for scalar, enum, object, and list arguments, including dynamic list rows and typed variable serialization
• improve detail-form interaction with per-argument enable/disable checkboxes, enter-to-edit text inputs, search within fields using /, and top/bottom navigation with gg and G
• improve panel navigation and clipboard support so users can scroll query and variable panels independently and copy content from the focused panel with Ctrl+y
• update endpoint filtering and detail rendering to better support multi-endpoint schemas, fragment expansion, and clearer focused-state styling
• allow single-file GraphQL mode to skip the kind: GraphQL check and rely only on the selected file and its URL, while keeping directory mode filtered by GraphQL kind
• add and expand tests for query building, variable rendering, schema parsing, dropdown behavior, and the single-file GraphQL validation path
• Adjust the layout with only one source. Top right height.