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.node.json

@@ -22,7 +22,13 @@
     "skipLibCheck": true,
     "noUnusedLocals": true,
     "noUnusedParameters": true,
-    "resolveJsonModule": true
+    "noFallthroughCasesInSwitch": true,
+    "resolveJsonModule": true,
+    // The build-tool configs include two hand-written .js files (tailwind/postcss). checkJs runs
+    // the type checker over them via their `@type` JSDoc, so a bad tailwind/postcss option is
+    // caught by the gate instead of silently mis-building CSS. Was the last hole in C-4.5.
+    "allowJs": true,
+    "checkJs": true
   },
-  "include": ["vite.config.ts", "vitest.config.ts"]
+  "include": ["vite.config.ts", "vitest.config.ts", "tailwind.config.js", "postcss.config.js"]
 }