build(frontend)!: knip drops --production; plain knip stays

The production pass (7ed2ac4e, E32) asked "is this export used by what
ships?". No real hit: its only findings were 37 exports a test imports,
answered with 36 `@internal` JSDoc tags (E34 S2) — test-only exports, not
defects. Since then it has only ever asked for another tag. The tags go
with it (36, no longer read by anything), and so do the production-only
`!…!` exclusions in knip.jsonc and their register rows. Plain knip (unused
files, exports and dependencies) keeps running in the gate.

frontend/src/lib/format.ts

@@ -60,7 +60,6 @@ export function formatSpeed(bps: number | null | undefined, lang: string): strin
  * Module-level because it is constant, and exported so the tests can hold that invariant. */
 // A non-empty tuple type (first element + rest) so `RELATIVE_UNITS[0]` is known-present under
 // noUncheckedIndexedAccess — the "sub-minute" branch relies on that first entry always existing.
-/** @internal exported for format.test.ts */
 export const RELATIVE_UNITS: readonly [
   [number, Intl.RelativeTimeFormatUnit],
   ...[number, Intl.RelativeTimeFormatUnit][],