Make dropdown the canonical site picker; remove broken click hint

Marimo's auto-attached chart_selection on a layered chart with multiple
data sources doesn't reliably surface clicked rows in chart1.value, so
the linked-detail panel never opened from a scatter click — only the
dropdown actually works. Stop telling the user to click a point: the
header text and the empty/multi-selection prompts now point to the
dropdown only. Legend bucket clicks (which do work) keep their callout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

notebook.py

@@ -123,9 +123,10 @@ def _(mo):
     ## Visual 1 — Win-win scatter with linked species breakdown
 
     Yield × total species richness, coloured by region, sized by coffee dominance
-    bucket. Median lines split the plot into four quadrants. **Click a point** to
-    see the species composition of that site below; **click a bucket** in the
-    dominance legend to filter to that management-intensity band.
+    bucket. Median lines split the plot into four quadrants. Use the **"Selected
+    site" dropdown** at the top to drill into a site's species composition (shown
+    below the scatter); **click a bucket** in the dominance legend to filter the
+    scatter to that management-intensity band.
     """)
     return
 
@@ -214,14 +215,14 @@ def _(alt, chart1, df, mo, pd, site_picker):
         sel = chart1.value  # pandas DataFrame of currently selected rows
     if sel is None or len(sel) == 0:
         detail = mo.md(
-            "*Click a point in the scatter above **or** pick one from the "
-            "\"Selected site\" dropdown at the top to see the site's species composition.*"
+            "*Pick a site from the **\"Selected site\" dropdown** at the top to see its "
+            "species composition.*"
         )
     elif len(sel) > 1:
         detail = mo.md(
             f"*{len(sel)} sites currently match the active filters. "
-            "Click a single point in the scatter above **or** pick one from the "
-            "\"Selected site\" dropdown to see its species composition.*"
+            "Pick one from the **\"Selected site\" dropdown** at the top to see its species "
+            "composition.*"
         )
     else:
         site = sel.iloc[0]