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/activeAccount.ts

@@ -10,7 +10,6 @@
 // again through `onAccountPinned`.
 //
 // ACTIVE_ACCOUNT_KEY is duplicated here rather than imported from api.ts so this stays dependency-free.
-/** @internal exported for 13 tests (BootGate.dom.test.tsx, …) */
 export const ACTIVE_ACCOUNT_KEY = "siftlode.activeAccount";
 
 // Re-readers for module-level per-account state, run each time the tab is pinned.