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/keymap.ts

@@ -258,7 +258,6 @@ function keyMatches(e: KeyboardEvent, name: string): boolean {
  *  Ctrl+A too, so select-all cycled the Plex audio track. It also means Shift+H is not `h` — a
  *  narrowing from the old `case "h": case "H":`, and the reason Shift chords can now be their own
  *  rows (Shift+← steps an episode without ← also seeking). */
-/** @internal exported for keymap.test.ts */
 export function matches(e: KeyboardEvent, chord: string): boolean {
   const parts = chord.split("+");
   const key = parts[parts.length - 1] ?? "";