[feat](trx-frontend-http): add F1 keyboard shortcuts overlay

Press F1 to toggle a help overlay listing available keyboard shortcuts.
Dismiss with F1, Escape, or clicking the backdrop. Refactored the
global keydown handler to route all shortcuts through one listener.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -9823,13 +9823,55 @@ function shouldIgnoreGlobalShortcut(target) {
   return !!target.closest("[contenteditable='true']");
 }
 
+// ── Shortcut help overlay ─────────────────────────────────────────────────────
+function toggleShortcutOverlay() {
+  const el = document.getElementById("shortcut-overlay");
+  if (!el) return;
+  el.classList.toggle("is-hidden");
+}
+function hideShortcutOverlay() {
+  const el = document.getElementById("shortcut-overlay");
+  if (el) el.classList.add("is-hidden");
+}
+function isShortcutOverlayVisible() {
+  const el = document.getElementById("shortcut-overlay");
+  return el && !el.classList.contains("is-hidden");
+}
+document.addEventListener("DOMContentLoaded", () => {
+  const overlay = document.getElementById("shortcut-overlay");
+  if (overlay) overlay.addEventListener("click", (e) => {
+    if (e.target === overlay) hideShortcutOverlay();
+  });
+});
+
 window.addEventListener("keydown", (event) => {
   if (event.defaultPrevented || event.repeat || event.isComposing) return;
+
+  const key = (event.key || "").toLowerCase();
+
+  // F1 — toggle shortcut help
+  if (event.key === "F1") {
+    event.preventDefault();
+    toggleShortcutOverlay();
+    return;
+  }
+
+  // Escape — close shortcut overlay if open
+  if (event.key === "Escape" && isShortcutOverlayVisible()) {
+    event.preventDefault();
+    hideShortcutOverlay();
+    return;
+  }
+
   if (event.ctrlKey || event.metaKey || event.altKey) return;
   if (shouldIgnoreGlobalShortcut(event.target)) return;
-  if ((event.key || "").toLowerCase() !== "s") return;
+
+  // S — spectrum screenshot
+  if (key === "s") {
     event.preventDefault();
     void captureSpectrumScreenshot();
+    return;
+  }
 }, { capture: true });
 
 // ── Zoom helpers ──────────────────────────────────────────────────────────────

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -1046,6 +1046,19 @@
       </div>
     </div>
   </div>
+  <div id="shortcut-overlay" class="shortcut-overlay is-hidden" aria-live="polite" aria-atomic="true">
+    <div class="shortcut-overlay-card">
+      <div class="shortcut-overlay-title">Keyboard Shortcuts</div>
+      <table class="shortcut-table">
+        <tbody>
+          <tr><td class="shortcut-key"><kbd>S</kbd></td><td>Spectrum screenshot</td></tr>
+          <tr><td class="shortcut-key"><kbd>F1</kbd></td><td>Toggle this help</td></tr>
+          <tr><td class="shortcut-key"><kbd>Esc</kbd></td><td>Close overlay / exit fullscreen</td></tr>
+        </tbody>
+      </table>
+      <div class="shortcut-overlay-hint">Press <kbd>F1</kbd> or <kbd>Esc</kbd> to close</div>
+    </div>
+  </div>
   <div id="decode-history-overlay" class="decode-history-overlay is-hidden" aria-live="polite" aria-atomic="true">
     <div class="decode-history-overlay-card">
       <div id="decode-history-overlay-title" class="decode-history-overlay-title">Loading decode history…</div>

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

@@ -1320,6 +1320,83 @@ small { color: var(--text-muted); }
 .sub-tab { flex-shrink: 0; background: transparent; border: none; border-bottom: 2px solid transparent; border-radius: 0; padding: 0.35rem 0.75rem; color: var(--text-muted); cursor: pointer; font-size: 0.85rem; height: auto; }
 .sub-tab.active { border-bottom-color: var(--accent-green); color: var(--accent-green); font-weight: 600; }
 .sub-tab:hover:not(.active) { color: var(--text); }
+/* ── Shortcut help overlay (F1) ─────────────────────────────────────── */
+.shortcut-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 9600;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 1.2rem;
+  background: color-mix(in srgb, var(--bg) 36%, transparent);
+  backdrop-filter: blur(6px);
+  -webkit-backdrop-filter: blur(6px);
+  opacity: 1;
+  visibility: visible;
+  transition: opacity 140ms ease, visibility 140ms ease;
+}
+.shortcut-overlay.is-hidden {
+  opacity: 0;
+  visibility: hidden;
+  pointer-events: none;
+}
+.shortcut-overlay-card {
+  min-width: min(22rem, calc(100vw - 2.4rem));
+  max-width: min(28rem, calc(100vw - 2.4rem));
+  padding: 1.2rem 1.4rem;
+  border-radius: 0.9rem;
+  border: 1px solid color-mix(in srgb, var(--border-light) 72%, transparent);
+  background: color-mix(in srgb, var(--card-bg) 92%, transparent);
+  box-shadow: 0 18px 40px rgba(0, 0, 0, 0.22);
+}
+.shortcut-overlay-title {
+  font-size: 1.05rem;
+  font-weight: 800;
+  color: var(--text-heading);
+  margin-bottom: 0.8rem;
+  text-align: center;
+}
+.shortcut-table {
+  width: 100%;
+  border-collapse: collapse;
+}
+.shortcut-table td {
+  padding: 0.32rem 0.4rem;
+  font-size: 0.88rem;
+  color: var(--text);
+  border-bottom: 1px solid color-mix(in srgb, var(--border-light) 40%, transparent);
+}
+.shortcut-table tr:last-child td {
+  border-bottom: none;
+}
+.shortcut-key {
+  width: 5rem;
+  text-align: right;
+  padding-right: 0.9rem !important;
+}
+.shortcut-key kbd,
+.shortcut-overlay-hint kbd {
+  display: inline-block;
+  min-width: 1.6em;
+  padding: 0.12em 0.45em;
+  border: 1px solid var(--border);
+  border-radius: 0.3rem;
+  background: var(--bg);
+  font-family: inherit;
+  font-size: 0.82rem;
+  font-weight: 600;
+  text-align: center;
+  color: var(--text);
+  box-shadow: 0 1px 0 color-mix(in srgb, var(--border) 60%, transparent);
+}
+.shortcut-overlay-hint {
+  margin-top: 0.7rem;
+  text-align: center;
+  font-size: 0.76rem;
+  color: var(--text-muted);
+}
+
 .decode-history-overlay {
   position: fixed;
   inset: 0;