fix(dashboard): Minor cleanup and improve text legibility (#7045)

**What**
- Cleans up some artifacts from the V1 -> V2 migrations.
- Removes the MedusaProvider from the root in favor of a plain QueryClient.
- Applies font styles to make the text in admin resemble designs in Figma more closely.

packages/admin-next/dashboard/src/vite-env.d.ts

@@ -1,7 +1,7 @@
 // / <reference types="vite/client" />
 
 interface ImportMetaEnv {
-  readonly MEDUSA_ADMIN_BACKEND_URL: string
+  readonly VITE_MEDUSA_ADMIN_BACKEND_URL: string
   readonly VITE_MEDUSA_V2: "true" | "false"
 }