[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/plugins/vchan.js

@@ -0,0 +1,170 @@
+// SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+// --- Virtual Channels Plugin ---
+//
+// Handles the `session` and `channels` SSE events emitted by /events and
+// provides the channel picker UI (SDR-only, shown when filter_controls is set).
+
+let vchanSessionId = null;
+let vchanRigId = null;
+let vchanChannels = [];
+let vchanActiveId = null;
+
+function vchanFmtFreq(hz) {
+  if (!Number.isFinite(hz) || hz <= 0) return "--";
+  if (hz >= 1e9) return (hz / 1e9).toFixed(4).replace(/\.?0+$/, "") + "\u202fGHz";
+  if (hz >= 1e6) return (hz / 1e6).toFixed(4).replace(/\.?0+$/, "") + "\u202fMHz";
+  if (hz >= 1e3) return (hz / 1e3).toFixed(1).replace(/\.?0+$/, "") + "\u202fkHz";
+  return hz + "\u202fHz";
+}
+
+// Called by app.js when the SSE `session` event arrives.
+function vchanHandleSession(data) {
+  try {
+    const d = JSON.parse(data);
+    vchanSessionId = d.session_id || null;
+  } catch (e) {
+    console.warn("vchan: bad session event", e);
+  }
+}
+
+// Called by app.js when the SSE `channels` event arrives.
+function vchanHandleChannels(data) {
+  try {
+    const d = JSON.parse(data);
+    vchanRigId = d.rig_id || null;
+    vchanChannels = d.channels || [];
+    // If the active channel was evicted, fall back to channel 0.
+    const ids = new Set(vchanChannels.map(c => c.id));
+    if (vchanActiveId && !ids.has(vchanActiveId)) {
+      vchanActiveId = vchanChannels.length > 0 ? vchanChannels[0].id : null;
+    }
+    vchanRender();
+  } catch (e) {
+    console.warn("vchan: bad channels event", e);
+  }
+}
+
+function vchanRender() {
+  const picker = document.getElementById("vchan-picker");
+  if (!picker) return;
+  picker.innerHTML = "";
+
+  vchanChannels.forEach(ch => {
+    const btn = document.createElement("button");
+    btn.type = "button";
+    btn.title = `Ch ${ch.index}: ${vchanFmtFreq(ch.freq_hz)} ${ch.mode} · ${ch.subscribers} subscriber${ch.subscribers !== 1 ? "s" : ""}`;
+    if (ch.id === vchanActiveId) btn.classList.add("active");
+
+    const label = document.createElement("span");
+    label.className = "vchan-label";
+    label.textContent = `${ch.index}: ${vchanFmtFreq(ch.freq_hz)} ${ch.mode}`;
+    btn.appendChild(label);
+
+    if (!ch.permanent) {
+      const del = document.createElement("span");
+      del.className = "vchan-del";
+      del.textContent = "\u00d7";
+      del.title = "Delete channel";
+      del.addEventListener("click", e => {
+        e.stopPropagation();
+        vchanDelete(ch.id);
+      });
+      btn.appendChild(del);
+    }
+
+    btn.addEventListener("click", () => {
+      if (ch.id !== vchanActiveId) vchanSubscribe(ch.id);
+    });
+
+    picker.appendChild(btn);
+  });
+
+  // "+" button — allocate a new channel at the current VFO frequency.
+  const addBtn = document.createElement("button");
+  addBtn.type = "button";
+  addBtn.className = "vchan-add";
+  addBtn.textContent = "+";
+  addBtn.title = "Allocate new virtual channel at current frequency";
+  addBtn.addEventListener("click", vchanAllocate);
+  picker.appendChild(addBtn);
+}
+
+async function vchanAllocate() {
+  if (!vchanSessionId || !vchanRigId) return;
+
+  // Use the last known rig frequency and mode as the starting point.
+  const freqHz = (typeof lastFreqHz === "number" && lastFreqHz > 0)
+    ? lastFreqHz
+    : 0;
+  const modeEl = document.getElementById("mode");
+  const mode = modeEl ? (modeEl.value || "USB") : "USB";
+
+  try {
+    const resp = await fetch(`/channels/${encodeURIComponent(vchanRigId)}`, {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ session_id: vchanSessionId, freq_hz: freqHz, mode }),
+    });
+    if (!resp.ok) {
+      const msg = await resp.text().catch(() => String(resp.status));
+      console.warn("vchan: allocate failed —", msg);
+      return;
+    }
+    const ch = await resp.json();
+    vchanActiveId = ch.id;
+    // The SSE `channels` event will trigger vchanRender(); optimistically
+    // mark active so the picker feels responsive even before the event arrives.
+    vchanRender();
+  } catch (e) {
+    console.error("vchan: allocate error", e);
+  }
+}
+
+async function vchanDelete(channelId) {
+  if (!vchanRigId) return;
+  try {
+    const resp = await fetch(
+      `/channels/${encodeURIComponent(vchanRigId)}/${encodeURIComponent(channelId)}`,
+      { method: "DELETE" }
+    );
+    if (!resp.ok) {
+      console.warn("vchan: delete failed", resp.status);
+    }
+    // Channel list updates via SSE `channels` event.
+  } catch (e) {
+    console.error("vchan: delete error", e);
+  }
+}
+
+async function vchanSubscribe(channelId) {
+  if (!vchanSessionId || !vchanRigId) return;
+  try {
+    const resp = await fetch(
+      `/channels/${encodeURIComponent(vchanRigId)}/${encodeURIComponent(channelId)}/subscribe`,
+      {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ session_id: vchanSessionId }),
+      }
+    );
+    if (!resp.ok) {
+      console.warn("vchan: subscribe failed", resp.status);
+      return;
+    }
+    vchanActiveId = channelId;
+    vchanRender();
+  } catch (e) {
+    console.error("vchan: subscribe error", e);
+  }
+}
+
+// Called by app.js from applyCapabilities().
+// Shows the channel picker only for SDR rigs.
+function vchanApplyCapabilities(caps) {
+  const row = document.getElementById("vchan-row");
+  if (!row) return;
+  row.style.display = (caps && caps.filter_controls) ? "" : "none";
+}