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/useFocusKeeper.ts

@@ -38,7 +38,6 @@ function firstFocusable(node: Node | null, fromEnd = false): HTMLElement | null
  *  else the previous one, widening to the parent's neighbours — so its removal does not drop the
  *  focus to <body>. For a removal the caller makes itself (a chip's ×) anywhere, including a modal
  *  or a side panel, where the keeper below does not watch. False when nothing was found. */
-/** @internal exported for useFocusKeeper.dom.test.tsx */
 export function focusNeighbour(el: HTMLElement): boolean {
   for (let at: HTMLElement | null = el; at && at !== document.body; at = at.parentElement) {
     const hit = firstFocusable(at.nextSibling) ?? firstFocusable(at.previousSibling, true);