Add viewer database import and clarify backup actions.

Pairs export with validated .db restore, documents the difference from game
save imports, and renames the JSON upload button to Import game backup.

Co-authored-by: Cursor <cursoragent@cursor.com>

static/style.css

@@ -1029,6 +1029,34 @@ tr.has-goal td:first-child { font-weight: 600; }
   margin-top: 8px;
 }
 
+.sidebar-backup-block {
+  margin-top: 12px;
+  padding-top: 12px;
+  border-top: 1px solid var(--border);
+}
+.sidebar-backup-label {
+  margin: 0 0 8px;
+  font-size: 0.72rem;
+  font-weight: 700;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--text-muted);
+}
+.sidebar-backup-help {
+  margin-top: 10px;
+  font-size: 0.78rem;
+  color: var(--text-muted);
+}
+.sidebar-backup-help summary {
+  cursor: pointer;
+  color: var(--accent);
+  font-weight: 600;
+}
+.sidebar-backup-help p {
+  margin: 8px 0 0;
+  line-height: 1.45;
+}
+
 .goal-modal {
   position: fixed;
   inset: 0;