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

tsconfig.json

@@ -1,6 +1,5 @@
 {
     "compilerOptions": {
-        "baseUrl": "./src",
         "target": "es2022",
         "lib": [
             "dom",
@@ -12,18 +11,20 @@
         "esModuleInterop": true,
         "allowSyntheticDefaultImports": true,
         "strict": false,
-        "downlevelIteration": true,
         "forceConsistentCasingInFileNames": true,
         "noFallthroughCasesInSwitch": false,
         "module": "ES2022",
-        "moduleResolution": "node",
+        "moduleResolution": "bundler",
         "resolveJsonModule": true,
         "isolatedModules": true,
         "noEmit": true,
         "jsx": "react-jsx",
         "paths": {
             "@layout/*": [
-                "layout/*"
+                "./src/layout/*"
+            ],
+            "@/*": [
+                "./src/*"
             ]
         }
     },