chore(vite): replace custom splitpanes types with @types/splitpanes

[adriandarian]

Description

Addresses the TODO in packages/vite/src/app/splitpanes.d.ts to install @types/splitpanes once updated.

Changes

• Added @types/splitpanes@^2.2.6 to the types catalog in pnpm-workspace.yaml
• Added @types/splitpanes as a dev dependency in packages/vite/package.json
• Removed the custom splitpanes.d.ts type declarations file

The official @types/splitpanes package is now available and maintained, so we can use it instead of maintaining custom type definitions.

Linked Issues

#9

Additional context