Add TypeScript 7 (tsgo) as fast type-checker alongside TS 6

TypeScript 7 is the Go-native rewrite of tsc, ~10x faster but only
distributed as @typescript/native-preview daily builds at the time
of writing (npm typescript@latest is still 6.0.3). Add it as a
non-disruptive type-check tool: yarn typecheck → tsgo --noEmit.

Vite still uses esbuild for transpilation, ESLint still uses TS 6
through @typescript-eslint v8, IDEs continue using their bundled TS.
This commit only adds a type-check tool — nothing replaces.

Required tsconfig.json adjustments for TS 7 compatibility (still
valid for TS 6):

- Drop baseUrl: "./src" (removed in TS 7). The codebase has no
  bare/non-relative imports that depended on it; all imports are
  relative or aliased.
- Drop downlevelIteration: true (removed in TS 7; target es2022
  doesn't need it).
- moduleResolution: "node" → "bundler" (TS 7 dropped node10; bundler
  is the right mode for Vite anyway).
- paths "@layout/*" entries now use leading "./" (TS 7 disallows
  non-relative path mappings). Add "@/*" → "./src/*" to match the
  Vite alias used in some components.

Other TS 7 adjustments:

- src/react-app-env.d.ts: add module declarations for *.css/.scss/.sass
  side-effect imports (TS 7 with bundler resolution requires them) +
  Window.NitroConfig / Window.NitroSecureApiUrl globals which were
  used in App.tsx without a declaration.
- src/common/Popover.tsx: explicit `import { JSX } from 'react'`
  because TS 7 dropped the implicit global JSX namespace.

Verification:
- yarn eslint still passes (TS 6 / @typescript-eslint v8 happy with
  the migrated config).
- yarn typecheck (tsgo) runs and reports only cascading errors
  rooted in the missing @nitrots/nitro-renderer sibling repo
  (environmental, not introduced here).

https://claude.ai/code/session_01GrR87LAqnAEyKG2ZbmQt5Q

package.json

@@ -8,7 +8,8 @@
         "start": "vite --host",
         "build": "vite build && node scripts/minify-dist.mjs",
         "build:prod": "npx browserslist@latest --update-db && yarn build",
-        "eslint": "eslint ./src"
+        "eslint": "eslint ./src",
+        "typecheck": "tsgo --noEmit"
     },
     "dependencies": {
         "@babel/runtime": "^7.29.2",
@@ -36,6 +37,7 @@
         "@types/react-dom": "^19.2.3",
         "@typescript-eslint/eslint-plugin": "^8.59.1",
         "@typescript-eslint/parser": "^8.59.1",
+        "@typescript/native-preview": "^7.0.0-dev.20260509.2",
         "@vitejs/plugin-react": "^6.0.1",
         "babel-plugin-react-compiler": "^1.0.0",
         "eslint": "^10.2.1",