[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/style.css

@@ -3330,3 +3330,130 @@ button:focus-visible, input:focus-visible, select:focus-visible {
   --wavelength-fg: #3a7a3a;
   --spectrum-bg: #e0f0e0;
 }
+
+/* =========================================================================
+   Scheduler tab
+   ========================================================================= */
+.sch-panel {
+  padding: 1rem;
+  max-width: 900px;
+}
+.sch-toast {
+  position: fixed;
+  bottom: 1.5rem;
+  left: 50%;
+  transform: translateX(-50%);
+  padding: 0.5rem 1.25rem;
+  border-radius: 0.4rem;
+  color: #fff;
+  font-weight: 600;
+  font-size: 0.9rem;
+  z-index: 999;
+  pointer-events: none;
+}
+.sch-row {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 1rem;
+  margin-bottom: 1rem;
+  align-items: flex-end;
+}
+.sch-label {
+  display: flex;
+  flex-direction: column;
+  gap: 0.3rem;
+  font-size: 0.85rem;
+  color: var(--text-muted);
+  font-weight: 600;
+  min-width: 9rem;
+}
+.sch-rig-select {
+  min-width: 10rem;
+}
+.sch-section {
+  margin-bottom: 1.5rem;
+  padding: 1rem;
+  background: var(--bg-secondary);
+  border-radius: 0.5rem;
+  border: 1px solid var(--border-light);
+}
+.sch-section-title {
+  font-size: 0.8rem;
+  font-weight: 700;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: var(--text-muted);
+  margin-bottom: 0.75rem;
+}
+.sch-ts-table {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 0.85rem;
+  margin-bottom: 1rem;
+}
+.sch-ts-table th,
+.sch-ts-table td {
+  padding: 0.4rem 0.6rem;
+  text-align: left;
+  border-bottom: 1px solid var(--border-light);
+}
+.sch-ts-table th {
+  color: var(--text-muted);
+  font-weight: 600;
+}
+.sch-remove-btn {
+  padding: 0.2rem 0.6rem;
+  background: var(--btn-bg);
+  color: var(--text);
+  border: 1px solid var(--border-light);
+  border-radius: 0.3rem;
+  cursor: pointer;
+  font-size: 0.8rem;
+}
+.sch-remove-btn:hover {
+  background: var(--btn-bg-hover, var(--btn-bg));
+}
+.sch-add-row {
+  align-items: flex-end;
+}
+.sch-actions {
+  display: flex;
+  gap: 0.75rem;
+  margin-bottom: 1.5rem;
+}
+.sch-save-btn {
+  background: var(--accent-green);
+  color: #fff;
+  border: none;
+  border-radius: 0.4rem;
+  padding: 0.5rem 1.25rem;
+  font-weight: 700;
+  cursor: pointer;
+  font-size: 0.95rem;
+}
+.sch-save-btn:disabled {
+  opacity: 0.5;
+  cursor: default;
+}
+.sch-reset-btn {
+  background: var(--btn-bg);
+  color: var(--text);
+  border: 1px solid var(--border-light);
+  border-radius: 0.4rem;
+  padding: 0.5rem 1.25rem;
+  font-weight: 600;
+  cursor: pointer;
+  font-size: 0.95rem;
+}
+.sch-status-card {
+  font-size: 0.9rem;
+  color: var(--text-muted);
+}
+@media (max-width: 600px) {
+  .sch-row {
+    flex-direction: column;
+  }
+  .sch-label {
+    min-width: 100%;
+  }
+}