refactor: migrate external deep imports to ui-server and broadcast-channel barrels...

refactor: migrate external deep imports to ui-server and broadcast-channel barrels (#1953)

Migrate the 2 external cross-sub-component deep imports enabled by the barrels introduced in #1952 to route through the barrel index files per the flat parent barrel convention.

Sites migrated (2):
- Bootstrap.ts:61 UIServerFactory — deep → ui-server/index.js
- ChargingStation.ts:114 ChargingStationWorkerBroadcastChannel —
  deep → broadcast-channel/index.js

Sites deliberately NOT migrated (documented in PR body):
- Bootstrap.ts:13 AbstractUIServer (type) — not re-exported; internal
  extension point per the ui-server barrel's `@file` omission list.
- ConfigurationSchema.ts:7 UIServerNet.isHostLiteralWithoutPort —
  UIServerNet not re-exported; internal helper per the omission list.
- AbstractUIService.ts:33 UIServiceWorkerBroadcastChannel (value) —
  adjacent sub-component crossing. Migrating triggers a circular-load
  ReferenceError at test time because the broadcast-channel barrel
  re-exports both concrete channels, transitively pulling
  ChargingStationWorkerBroadcastChannel's charging-station chain back
  through ui-server before class declarations complete.
- UIServiceWorkerBroadcastChannel.ts:1 AbstractUIService (type-only) —
  adjacent sub-component crossing; kept direct for symmetry.

Convention codified: barrels are for CROSS-COMPONENT external
consumers, not for adjacent-sub-component crossings within the same
parent module. Matches the meter-values/index.ts convention.

Quality gates: typecheck clean, lint clean, 2955/2961 tests pass, 0 fail.

Diff: +2/-2 across 2 files.

Follow-up to #1952 (barrel gaps closure). PR-D' from the sequence
tracked in #1950's scope fence.