[feat](trx-frontend-http): virtual channel manager and picker UI

Add client-side virtual channel support (Phase 1 — metadata only):

- vchan.rs: ClientChannelManager keyed by rig_id; tracks per-session
  channel subscriptions and broadcasts list changes via change_tx
- server.rs: instantiate Arc<ClientChannelManager> and expose as app_data
- api.rs: wire ClientChannelManager into /events SSE (session UUID,
  init_rig, update_primary, channel change stream, session cleanup on
  disconnect); add channel CRUD routes:
    GET/POST /channels/{rig_id}
    DELETE   /channels/{rig_id}/{channel_id}
    POST     /channels/{rig_id}/{channel_id}/subscribe
    PUT      /channels/{rig_id}/{channel_id}/freq|mode
- auth.rs: classify /channels/ prefix as Read access
- plugins/vchan.js: channel picker with +/× buttons, subscribe on click,
  SDR-only (shown when filter_controls capability is set)
- app.js: handle SSE `session` and `channels` events, call
  vchanApplyCapabilities from applyCapabilities
- index.html: #vchan-row div + <script src="/vchan.js">
- style.css: .vchan-picker pill styles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -2598,6 +2598,7 @@ dependencies = [
  "trx-core",
  "trx-frontend",
  "trx-protocol",
+ "uuid",
 ]
 
 [[package]]