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/components/channelColumns.tsx

@@ -45,7 +45,6 @@ export function applySyncFilter(rows: ManagedChannel[], value: unknown): Managed
 /** The Tags column filter predicate (item B): the "No tag" sentinel keeps channels the user hasn't
  *  personally tagged (empty `tag_ids`); it's exclusive with real tag picks (the chip row enforces that),
  *  so when present it wins. Otherwise a channel passes if it carries ANY selected tag id. */
-/** @internal exported for channelColumns.dom.test.tsx */
 export function tagsColumnTest(c: ManagedChannel, values: string[]): boolean {
   if (values.includes(NO_TAG_SENTINEL)) return c.tag_ids.length === 0;
   return values.some((v) => c.tag_ids.includes(Number(v)));