Add species-level data fallback on cultivar detail page

When cultivar fields are empty, show species-level data as fallback with
italic styling and a "~ species" indicator. Applied to: frost tolerance
(from USDA zone), light requirement (inferred from plant layer), and
climate card (drought tolerance, USDA hardiness zone). Includes a footer
note explaining estimated values.

herbapi-ui/assets/herbapi.css

@@ -1197,6 +1197,28 @@ td.placeholder {
     text-decoration: underline;
 }
 
+/* Species-fallback estimated values */
+
+.estimated-value {
+    color: var(--accent);
+    font-style: italic;
+}
+
+.estimated-tag {
+    font-size: 0.7rem;
+    color: var(--text-muted);
+    margin-left: 0.4rem;
+    opacity: 0.7;
+}
+
+.species-fallback-note {
+    font-size: 0.8rem;
+    color: var(--text-muted);
+    font-style: italic;
+    margin-top: 1.25rem;
+    opacity: 0.7;
+}
+
 /* Responsive */
 
 @media (max-width: 768px) {