build(frontend): enable the zero-fallout compiler flags (S3a)

Three strictness flags that the codebase already satisfies (0 errors each):
- noFallthroughCasesInSwitch on src + e2e — a missing `break` is now a type error.
- noUnusedLocals + noUnusedParameters on the e2e project, so it matches src and node
  and dead spec scaffolding can't accumulate (it was the one project without them).
- allowJs + checkJs on the node project, pulling tailwind.config.js and
  postcss.config.js under the type checker (the last hole in C-4.5) — a real type
  error in either is now caught (verified: `content: 123` fails; Config stays
  permissive on plugin keys by design).

noUncheckedIndexedAccess is NOT here — it has 98 call-site fixes and lands as its own
reviewable sub-sprint (S3b).

frontend/tsconfig.json

@@ -13,7 +13,8 @@
     "jsx": "react-jsx",
     "strict": true,
     "noUnusedLocals": true,
-    "noUnusedParameters": true
+    "noUnusedParameters": true,
+    "noFallthroughCasesInSwitch": true
   },
   "include": ["src"],
   // Editors resolve a file to the nearest tsconfig that INCLUDES it; vite.config.ts / vitest.config.ts