feat(ui): add help modal and copy terminal button

- Add floating help button (?) with keyboard shortcuts modal
- Add Ctrl+? shortcut to open help modal
- Add copy terminal button to copy output to clipboard
- Style kbd elements for keyboard shortcut display
- Update Escape key to close all modals

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/web/public/styles.css

@@ -1694,3 +1694,75 @@ input[type="checkbox"] {
   align-items: center;
   gap: 0.5rem;
 }
+
+/* Keyboard Shortcuts Modal */
+.shortcuts-list {
+  display: flex;
+  flex-direction: column;
+  gap: 1.5rem;
+}
+
+.shortcut-group h4 {
+  font-size: 0.75rem;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  color: var(--text-muted);
+  margin-bottom: 0.75rem;
+  padding-bottom: 0.5rem;
+  border-bottom: 1px solid var(--border);
+}
+
+.shortcut-row {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 0.5rem 0;
+  font-size: 0.875rem;
+}
+
+.shortcut-row span {
+  color: var(--text-dim);
+}
+
+kbd {
+  display: inline-block;
+  padding: 0.25rem 0.5rem;
+  background: linear-gradient(135deg, #2a2a2a 0%, #1a1a1a 100%);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  font-family: 'SF Mono', Monaco, monospace;
+  font-size: 0.75rem;
+  color: var(--text);
+  box-shadow: 0 2px 0 var(--border);
+  min-width: 24px;
+  text-align: center;
+}
+
+/* Help button */
+.help-btn {
+  position: fixed;
+  bottom: 1rem;
+  right: 1rem;
+  width: 40px;
+  height: 40px;
+  border-radius: 50%;
+  background: linear-gradient(135deg, #2a2a2a 0%, #1a1a1a 100%);
+  border: 1px solid var(--border);
+  color: var(--text-dim);
+  font-size: 1.25rem;
+  cursor: pointer;
+  z-index: 100;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition: all 0.2s ease;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
+}
+
+.help-btn:hover {
+  background: linear-gradient(135deg, #3a3a3a 0%, #2a2a2a 100%);
+  color: var(--text);
+  transform: translateY(-2px);
+  box-shadow: 0 6px 16px rgba(0, 0, 0, 0.4);
+}