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/notifications.ts

@@ -95,7 +95,6 @@ const ERROR_TTL = 15000;
 // A toast that carries an action (an "Undo") lives at least this long, whatever the reader's toast
 // time: its button is the only way back, and a 3 s toast leaves no time to reach it (owner, E28).
 // The setting still decides for longer, and "never" still means never.
-/** @internal exported for notifications.actionTtl.dom.test.ts */
 export const ACTION_TTL_MIN = 8000;
 
 // 6b: per-account notification settings (persisted by the Settings panel into the