feat: Smart Folders sidebar, category heatmap, sidebar counts API

- Replace plain activity heatmap with CategoryHeatmapWidget on dashboard
- Add collapsible Smart Folders sidebar with Favorites, Mailbox, Smart Folders sections
- Favorites: customisable pinned shortcuts (default: Inbox, Unread, Large, Old, Read Later); pin/unpin smart folders via buttons; persists to localStorage
- Mailbox section: 11 Gmail mailbox items with icons and live count badges
- Smart Folders section: 10 category folders sorted by count desc
- Live count badges via new GET /api/v1/analytics/sidebar-counts endpoint
- Backend: SidebarCountsDto, GetSidebarCountsAsync with label lookups for SENT/DRAFT/SPAM, size/age filters, EmailCategory mapping
- Sidebar collapses to icon-only; section states persist to localStorage

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

frontend/src/styles.css

@@ -19,7 +19,73 @@ body { margin: 0; font-family: system-ui, -apple-system, Segoe UI, Roboto, sans-
 .spacer { flex: 1; }
 .user { color: var(--muted); font-size: 13px; }
 
-main { padding: 20px; }
+/* ── App body (sidebar + main) ── */
+.app-body { display: flex; min-height: calc(100vh - 53px); }
+
+.sidebar {
+  width: 216px; flex-shrink: 0;
+  background: var(--panel); border-right: 1px solid #2c3550;
+  display: flex; flex-direction: column; overflow-y: auto;
+  transition: width 0.2s ease;
+}
+.sidebar--collapsed { width: 48px; }
+
+.sidebar-header {
+  display: flex; align-items: center; justify-content: space-between;
+  padding: 12px 8px 10px;
+  border-bottom: 1px solid #2c3550;
+  min-height: 42px; flex-shrink: 0;
+}
+.sidebar-brand { font-size: 12px; font-weight: 700; letter-spacing: 0.05em; text-transform: uppercase; color: var(--muted); padding-left: 6px; }
+.sidebar-toggle { padding: 2px 6px; font-size: 17px; line-height: 1; color: var(--muted); flex-shrink: 0; }
+.sidebar-toggle:hover { color: var(--text); }
+
+/* Section headings */
+.section-head {
+  display: flex; align-items: center; justify-content: space-between;
+  width: 100%; background: none; border: none; border-bottom: 1px solid #2c3550;
+  padding: 7px 10px 7px 12px; cursor: pointer;
+  color: var(--muted); font-size: 10px; font-weight: 700; letter-spacing: 0.07em; text-transform: uppercase;
+}
+.section-head:hover { color: var(--text); }
+.section-title { flex: 1; text-align: left; white-space: nowrap; overflow: hidden; }
+.section-arrow { font-size: 13px; transform: rotate(90deg); display: inline-block; transition: transform 0.15s; }
+.section-arrow--closed { transform: rotate(0deg); }
+
+/* Folder items */
+.folder-list { list-style: none; margin: 0; padding: 2px 0; }
+.folder-item {
+  display: flex; align-items: center; gap: 8px;
+  padding: 6px 10px 6px 12px;
+  color: var(--muted); text-decoration: none; font-size: 13px;
+  border-left: 3px solid transparent;
+  white-space: nowrap; overflow: hidden;
+  transition: background 0.1s, color 0.1s;
+}
+.folder-item:hover { background: var(--panel-2); color: var(--text); }
+.folder-item:hover .fav-pin { opacity: 1; }
+.folder-item--active { border-left-color: var(--accent); color: var(--text); background: var(--panel-2); }
+.folder-icon { font-size: 14px; flex-shrink: 0; }
+.folder-label { flex: 1; overflow: hidden; text-overflow: ellipsis; }
+.folder-spacer { flex: 1; }
+.folder-badge {
+  font-size: 10px; font-weight: 600; color: var(--muted);
+  background: var(--panel-2); border-radius: 8px; padding: 1px 5px;
+  flex-shrink: 0;
+}
+.folder-item--active .folder-badge { background: #2c3550; color: var(--accent); }
+
+/* Favorite pin/unpin button */
+.fav-pin {
+  background: none; border: none; padding: 0 2px; cursor: pointer;
+  font-size: 12px; color: var(--muted); opacity: 0;
+  transition: opacity 0.1s, color 0.1s; flex-shrink: 0; line-height: 1;
+}
+.fav-pin:hover { color: var(--accent); }
+.fav-pin--remove { opacity: 1; color: var(--danger); }
+.fav-pin--remove:hover { color: #ff7070; }
+
+main { padding: 20px; flex: 1; min-width: 0; }
 
 button, .btn { background: var(--accent); color: #fff; border: none; border-radius: 6px; padding: 8px 14px; cursor: pointer; text-decoration: none; font-size: 14px; }
 button.danger, .btn.danger { background: var(--danger); }
@@ -54,6 +120,43 @@ table.grid th, table.grid td { padding: 8px; border-bottom: 1px solid #2c3550; t
 .hm-day { width: 30px; font-size: 10px; color: var(--muted); }
 .hm-cell { width: 10px; height: 10px; background: var(--accent); border-radius: 2px; }
 
+/* Category heatmap */
+.cat-heatmap { display: flex; flex-direction: column; gap: 3px; overflow: auto; }
+.chm-row { display: grid; grid-template-columns: 92px repeat(7, 1fr); gap: 3px; align-items: stretch; }
+.chm-head .chm-col { font-size: 10px; color: var(--muted); text-align: center; }
+.chm-label { font-size: 11px; color: var(--muted); overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+.chm-cell { background: var(--accent); border-radius: 3px; min-height: 22px; display: flex; align-items: center; justify-content: center; font-size: 10px; color: #fff; }
+
+/* react-grid-layout resize handle — make it clearly visible on the dark theme */
+.react-resizable-handle {
+  opacity: 0; transition: opacity 0.15s;
+}
+.grid-item:hover .react-resizable-handle { opacity: 1; }
+.react-resizable-handle::after {
+  content: ''; position: absolute; right: 4px; bottom: 4px;
+  width: 9px; height: 9px;
+  border-right: 2px solid var(--accent); border-bottom: 2px solid var(--accent);
+  border-bottom-right-radius: 2px;
+}
+.react-grid-item.react-grid-placeholder { background: var(--accent); opacity: 0.25; border-radius: 10px; }
+
+/* Senders by volume */
+.senders-page { max-width: 1100px; }
+.tier { margin-top: 22px; }
+.tier-head { display: flex; align-items: center; gap: 10px; margin-bottom: 10px; }
+.tier-head h3 { margin: 0; font-size: 16px; }
+.tier-dot { width: 12px; height: 12px; border-radius: 50%; }
+.tier-count { color: var(--muted); font-size: 12px; }
+.sender-blocks { display: grid; grid-template-columns: repeat(auto-fill, minmax(190px, 1fr)); gap: 10px; }
+.sender-block { background: var(--panel); border: 1px solid #2c3550; border-top: 3px solid var(--accent); border-radius: 8px; padding: 10px 12px; }
+.sb-name { font-weight: 600; font-size: 13px; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
+.sb-domain { color: var(--muted); font-size: 11px; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; margin-bottom: 6px; }
+.sb-stats { display: flex; align-items: baseline; gap: 8px; flex-wrap: wrap; }
+.sb-count { font-size: 20px; font-weight: 700; }
+.sb-unread { font-size: 11px; color: var(--accent); }
+.sb-unsub { font-size: 10px; color: var(--muted); border: 1px solid #36405c; border-radius: 4px; padding: 0 4px; }
+.tier-more { margin-top: 8px; }
+
 .page { max-width: 900px; }
 .form-row { display: flex; gap: 10px; margin: 14px 0; }
 .form-row input { flex: 1; }