Fix IndexError when selecting search results

The search header was being added to ListView but not to message_store.envelopes,
causing an index mismatch when marking messages as read. Now the search header
is included in the envelopes list and metadata_by_id is properly updated so
indices align between ListView and the message store.

src/mail/app.py

@@ -1002,15 +1002,15 @@ class EmailViewerApp(App):
 
         config = get_config()
 
-        # Add search results header
+        # Build search header label
         if results:
             header_label = f"Search: '{query}' ({len(results)} result{'s' if len(results) != 1 else ''})"
         else:
             header_label = f"Search: '{query}' - No results found"
-        envelopes_list.append(ListItem(GroupHeader(label=header_label)))
 
         if not results:
             # Clear the message viewer when no results
+            envelopes_list.append(ListItem(GroupHeader(label=header_label)))
             content_container = self.query_one(ContentContainer)
             content_container.clear_content()
             self.message_store.envelopes = []
@@ -1019,14 +1019,64 @@ class EmailViewerApp(App):
             return
 
         # Create a temporary message store for search results
+        # We need to include the search header in the envelopes so indices match
         search_store = MessageStore()
-        search_store.load(results, self.sort_order_ascending)
 
-        # Store for navigation (replace main store temporarily)
+        # Manually build envelopes list with search header first
+        # so that ListView indices match message_store.envelopes indices
+        grouped_envelopes = [{"type": "header", "label": header_label}]
+
+        # Sort results by date
+        sorted_results = sorted(
+            results,
+            key=lambda x: x.get("date", ""),
+            reverse=not self.sort_order_ascending,
+        )
+
+        # Group by month and build metadata
+        months: Dict[str, bool] = {}
+        for envelope in sorted_results:
+            if "id" not in envelope:
+                continue
+
+            # Extract date and determine month group
+            date_str = envelope.get("date", "")
+            try:
+                date = datetime.fromisoformat(date_str.replace("Z", "+00:00"))
+                month_key = date.strftime("%B %Y")
+            except (ValueError, TypeError):
+                month_key = "Unknown Date"
+
+            # Add month header if this is a new month
+            if month_key not in months:
+                months[month_key] = True
+                grouped_envelopes.append({"type": "header", "label": month_key})
+
+            # Add the envelope
+            grouped_envelopes.append(envelope)
+
+            # Store metadata for quick access (index matches grouped_envelopes)
+            envelope_id = int(envelope["id"])
+            search_store.metadata_by_id[envelope_id] = {
+                "id": envelope_id,
+                "subject": envelope.get("subject", ""),
+                "from": envelope.get("from", {}),
+                "to": envelope.get("to", {}),
+                "cc": envelope.get("cc", {}),
+                "date": date_str,
+                "index": len(grouped_envelopes) - 1,
+            }
+
+        search_store.envelopes = grouped_envelopes
+        search_store.total_messages = len(search_store.metadata_by_id)
+
+        # Store for navigation (replace main store)
         self.message_store.envelopes = search_store.envelopes
+        self.message_store.metadata_by_id = search_store.metadata_by_id
         self.total_messages = len(results)
 
-        for item in search_store.envelopes:
+        # Build ListView to match envelopes list exactly
+        for item in self.message_store.envelopes:
             if item and item.get("type") == "header":
                 envelopes_list.append(ListItem(GroupHeader(label=item["label"])))
             elif item: