refactor: rename Demo components to canonical names and purge dead code

- DashboardDemo → Dashboard, PeriodSelectorDemo → Comparison (these were the real active routes)
- Delete old Dashboard, Comparison, NavDemo, Slides, ChartExport (replaced / unused)
- Delete 8 unused shared components: DateRangePicker, PeriodPicker, FilterControls, MultiSelect, Carousel, ChartCard, EmptyState, StatCard, ToggleSwitch
- Fix date picker stay-open behavior: selections now update draft state only; Apply/Cancel buttons commit or discard
- shared/index.tsx now only exports LoadingSkeleton

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.css

@@ -116,6 +116,16 @@
   box-sizing: border-box;
 }
 
+/* Prevent horizontal overflow at the root level */
+html, body {
+  overflow-x: hidden;
+}
+
+main {
+  min-width: 0;
+  overflow-x: hidden;
+}
+
 body {
   font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
   background: var(--bg);
@@ -1089,6 +1099,41 @@ table tbody tr:hover {
   flex-shrink: 0;
 }
 
+.drp-footer {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+}
+
+.drp-cancel, .drp-apply {
+  padding: 7px 16px;
+  border-radius: 7px;
+  font-size: 0.825rem;
+  font-weight: 600;
+  cursor: pointer;
+  transition: background 0.12s, color 0.12s;
+}
+
+.drp-cancel {
+  background: transparent;
+  border: 1px solid var(--border);
+  color: var(--text-secondary);
+}
+
+.drp-cancel:hover {
+  background: var(--bg-secondary);
+}
+
+.drp-apply {
+  background: var(--accent);
+  border: 1px solid transparent;
+  color: #fff;
+}
+
+.drp-apply:hover {
+  opacity: 0.88;
+}
+
 /* Right-align panel when near viewport edge */
 @media (max-width: 480px) {
   .drp-panel {