[feat](trx-frontend-http): add Background Decoding Scheduler

Implements a scheduler that retunes the rig automatically when no SSE
clients are connected.  Two modes are supported:

- Grayline: tunes to per-period bookmarks (dawn/day/dusk/night) based on
  an inline NOAA solar algorithm given station lat/lon.
- Time Span: tunes to bookmarks within user-defined UTC windows; midnight-
  spanning intervals supported.

Backend:
- SchedulerStore (PickleDB, sch:{rig_id} keys) in scheduler.rs
- spawn_scheduler_task polls every 30 s, checks context.sse_clients == 0,
  sends SetFreq + SetMode via RigRequest with rig_id_override
- HTTP API: GET/PUT/DELETE /scheduler/{rig_id}, GET …/status
- sse_clients Arc<AtomicUsize> added to FrontendRuntimeContext and shared
  with the SSE counter in build_server (single source of truth)
- /scheduler/ added to Read auth routes (write requires Control)

Frontend:
- Scheduler tab (clock icon, 6th position) with Grayline/TimeSpan UI
- scheduler.js plugin: loads config + bookmarks, live status polling
  every 15 s, write controls hidden for Rx-role users
- CSS .sch-* component styles added to style.css
- SCHEDULER.md design document at repo root

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/app.js

@@ -3347,7 +3347,7 @@ if (spectrumBwSweetBtn) {
 }
 
 // --- Tab navigation ---
-const TAB_ORDER = ["main", "bookmarks", "decoders", "map", "about"];
+const TAB_ORDER = ["main", "bookmarks", "decoders", "map", "scheduler", "about"];
 
 function navigateToTab(name) {
   if (authEnabled && !authRole && name !== "main") return;
@@ -3415,6 +3415,10 @@ document.querySelector(".tab-bar").addEventListener("click", (e) => {
 window.addEventListener("resize", () => { scheduleSpectrumLayout(); });
 
 // --- Auth startup sequence ---
+function getAvailableRigIds() {
+  return lastRigIds || [];
+}
+
 async function initializeApp() {
   showAuthGate(false);
   const authStatus = await checkAuthStatus();
@@ -3426,6 +3430,7 @@ async function initializeApp() {
     updateAuthUI();
     connect();
     connectDecode();
+    initSchedulerUI();
     resizeHeaderSignalCanvas();
     startHeaderSignalSampling();
     return;
@@ -3439,6 +3444,7 @@ async function initializeApp() {
     applyAuthRestrictions();
     connect();
     connectDecode();
+    initSchedulerUI();
     resizeHeaderSignalCanvas();
     startHeaderSignalSampling();
   } else {
@@ -3449,6 +3455,13 @@ async function initializeApp() {
   }
 }
 
+function initSchedulerUI() {
+  if (typeof initScheduler === "function") {
+    initScheduler(lastActiveRigId, authRole);
+    wireSchedulerEvents();
+  }
+}
+
 // Setup auth form
 document.getElementById("auth-form").addEventListener("submit", async (e) => {
   e.preventDefault();
@@ -3466,6 +3479,7 @@ document.getElementById("auth-form").addEventListener("submit", async (e) => {
     applyAuthRestrictions();
     connect();
     connectDecode();
+    initSchedulerUI();
     resizeHeaderSignalCanvas();
     startHeaderSignalSampling();
   } catch (err) {
@@ -3488,6 +3502,7 @@ if (guestBtn) {
     applyAuthRestrictions();
     connect();
     connectDecode();
+    initSchedulerUI();
     resizeHeaderSignalCanvas();
     startHeaderSignalSampling();
   });