Config file with defineConfig() for gpjsui package

[tom96da]

`gpjsui package` currently reads app metadata (productName, identifier,
icon) from package.json's own fields plus a "gpjsui" key — plain JSON,
no code. Want a proper `gpjsui.config.mts` with a `defineConfig()` helper
instead, the way Vite/Vitest do it, once there's more than a handful of
config fields.

Blocked on: `@gpjs-ui/cli` doesn't depend on `vite` directly (only
`@gpjs-ui/vite` does), and Node 20 — still in the CI test matrix — can't
load `.mts` natively. Evaluating a config file needs either borrowing
Vite's own config loader or extending the Bundler contract to carry
config loading too.

Out of scope

• Changing how dev/build/package read metadata today — package.json's
  "gpjsui" key stays until this lands

Tasks

• Decide how a `.mts` config gets evaluated without adding a second
  vite dependency or a second config-loading mechanism
• `defineConfig()` helper with real types for the metadata gpjsui
  package already reads
• `gpjsui.config.mts` wins over package.json's "gpjsui" key when both exist