test(frontend): lock the style invariants a QA pass keeps re-finding

Two defects in this QA pass were pure CSS drift with no test surface at all:
100vw full-bleed sections panning the document by a scrollbar's width, and
controls sitting on three different corner radii. Assert both against the
sources, so the next person who hard-codes a radius or drops an overflow-x
guard fails CI instead of failing a screenshot review.

Tests now read files off disk, so they live in their own tsc project
(tsconfig.test.json, node types) excluded from the browser build.

frontend/tsconfig.test.json

@@ -0,0 +1,27 @@
+{
+  "compilerOptions": {
+    "target": "es2023",
+    "lib": ["ES2023", "DOM"],
+    "module": "esnext",
+    // Tests read source files off disk (the style-invariant test), so they need
+    // the Node types the browser build deliberately excludes.
+    "types": ["node", "vite/client"],
+    "allowArbitraryExtensions": true,
+    "skipLibCheck": true,
+
+    /* Bundler mode */
+    "moduleResolution": "bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "jsx": "react-jsx",
+
+    /* Linting */
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "erasableSyntaxOnly": true,
+    "noFallthroughCasesInSwitch": true
+  },
+  "include": ["src/**/tests/*", "src/test"]
+}