build(frontend): knip also checks what ships; strict indexing in every project

`npm run knip` now runs a second, --production pass, which asks whether the
code that ships uses an export, not just whether anything does. Of the 47
exports only tests reached:
- 34 are exported for their test and say so with @internal, naming the test.
- 11 were module-private names with a stray `export`. Two of them were not
  used at all and are deleted.
- The unused alreadyReported re-export is gone.
Test kits, the e2e suite and the two build-tooling modules are excluded from
the production project, each named in knip.jsonc.

noUncheckedIndexedAccess is now on for the node and e2e tsconfigs too (0
errors), matching the app project. tsconfig.node.json listed a
tailwind.config.js that has not existed since Tailwind 4. That entry is now
playwright.config.ts, which the typed lint finds through this project.

frontend/src/lib/colorMath.ts

@@ -34,6 +34,7 @@ export function hueGap(a: string, b: string): number {
 
 /** `color-mix(in srgb, a w, b)` for two opaque hex colours: the channels mix gamma-encoded, as the
  *  browser does in the srgb space. */
+/** @internal exported for theme.contract.test.ts */
 export function mixHex(a: string, b: string, w: number): string {
   const [x, y] = [rgb(a), rgb(b)];
   return `#${x