Add bottle size, bourbon list modal, and stat improvements

- Add bottle_size field to User model and UserResponse/UserUpdate schemas
- Settings modal includes bottle size input (shots capacity)
- Community bottles and My Bottle page show fill bar based on bottle size
- Community bottle cards are clickable — opens searchable bourbon list modal
- Add total_shots_added stat to replace duplicate net volume on dashboard
- Reorder dashboard stats: Bourbons Added, Total Poured In, Shots Remaining, Est. Proof
- Theme-matched custom scrollbar (amber on dark)

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

frontend/css/style.css

@@ -475,6 +475,12 @@ tbody td {
 .tab.active, .tab:hover { color: var(--amber-light); }
 .tab.active { border-bottom-color: var(--amber); }
 
+/* ---- Custom scrollbar ---- */
+::-webkit-scrollbar { width: 8px; }
+::-webkit-scrollbar-track { background: var(--bg-card); }
+::-webkit-scrollbar-thumb { background: var(--amber-dim); border-radius: 4px; }
+::-webkit-scrollbar-thumb:hover { background: var(--amber); }
+
 /* ---- Responsive ---- */
 @media (max-width: 640px) {
   .hero h1 { font-size: 2rem; }