[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>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/style.css

@@ -365,6 +365,43 @@ input.status-input, select.status-input { width: 100%; padding: 0.45rem 0.5rem;
   background: var(--btn-bg);
   font-weight: 600;
 }
+.vchan-picker {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+}
+.vchan-picker button {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  border: 1px solid var(--border-light);
+  border-radius: 6px;
+  height: var(--control-height);
+  padding: 0 0.55rem;
+  background: var(--input-bg);
+  color: var(--text-muted);
+  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
+  font-size: 0.82rem;
+  cursor: pointer;
+}
+.vchan-picker button.active {
+  background: var(--btn-bg);
+  color: var(--text);
+  font-weight: 600;
+  border-color: var(--btn-border, var(--border-light));
+}
+.vchan-del {
+  opacity: 0.5;
+  font-size: 1rem;
+  line-height: 1;
+}
+.vchan-del:hover { opacity: 1; }
+.vchan-add {
+  font-size: 1.1rem;
+  font-weight: 700;
+  padding: 0 0.6rem !important;
+  color: var(--text-muted);
+}
 .signal-measure {
   display: inline-flex;
   gap: 0.5rem;