Add postgres schema query to ElectronAPI

Related to #3 Add IPC function to query PostgreSQL schema. * **src/main.ts** - Import `Client` from `pg`. - Add IPC handler `get-pg-schema` to connect to PostgreSQL, execute the specified query, and return the schema information. * **src/preload.ts** - Add `getPgSchema` function to the ElectronAPI to invoke the `get-pg-schema` IPC handler. * **src/types.ts** - Add `getPgSchema` function definition to the ElectronAPI interface. --- For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/williamcotton/guish/issues/3?shareId=3708c57f-5b64-40a1-837a-89e3f16e1a7e).