Improve and refactor API client drastically

[mike1858]

This PR improves the type safety of the frontend API client, reducing type errors across the frontend. It should also fix #29.

[mike1858]

@bl-ue will you please review?

[bl-ue]

Linting errors.

> gemini-desktop@0.1.0 lint:ci /home/runner/work/gemini-desktop/gemini-desktop/frontend
> pnpm eslint src --max-warnings=0 && pnpm tsc --noEmit

Error: src/components/common/DirectorySelectionDialog.tsx(87,59): error TS2345: Argument of type 'string' is not assignable to parameter of type '{ path: string; }'.
Error: src/components/common/DirectorySelectionDialog.tsx(121,55): error TS2345: Argument of type 'string' is not assignable to parameter of type '{ path: string; }'.
Error: src/hooks/useFileSystemNavigation.ts(47,57): error TS2345: Argument of type 'string' is not assignable to parameter of type '{ path: string; }'.
Error: src/hooks/useFileSystemNavigation.ts(118,59): error TS2345: Argument of type 'string' is not assignable to parameter of type '{ path: string; }'.