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.app.json

@@ -22,5 +22,7 @@
     "erasableSyntaxOnly": true,
     "noFallthroughCasesInSwitch": true
   },
-  "include": ["src"]
+  "include": ["src"],
+  // Tests are a separate project (tsconfig.test.json): they need Node types.
+  "exclude": ["src/**/tests/*", "src/test"]
 }