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.json

@@ -2,6 +2,7 @@
   "files": [],
   "references": [
     { "path": "./tsconfig.app.json" },
+    { "path": "./tsconfig.test.json" },
     { "path": "./tsconfig.node.json" }
   ]
 }