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/playerModalHooks.ts

@@ -36,7 +36,6 @@ const MAX_CONSECUTIVE_ERRORS = 3;
 export const YT_API_LOAD_FAILED = -2;
 /** How long the script may take before the player gives up on it. The real answer comes in well
  *  under a second; anything near this is a blocked or stalled request, not a slow one. */
-/** @internal exported for playerModalHooks.apiload.dom.test.tsx */
 export const YT_API_TIMEOUT_MS = 15_000;
 let apiPromise: Promise<YTNamespace> | null = null;
 function loadYouTubeApi(): Promise<YTNamespace> {