Add Himalaya search integration tests and fix 0 results display

- Add test mailbox with 5 sample emails for integration testing
- Add himalaya_test_config.toml for local Maildir backend testing
- Create 12 integration tests covering search by from/to/subject/body
- Fix search results display to clear list and show message when 0 results
- Add clear_content() method to ContentContainer widget

src/mail/app.py

@@ -1003,9 +1003,21 @@ class EmailViewerApp(App):
         config = get_config()
 
         # Add search results header
-        header_label = f"Search: '{query}' ({len(results)} result{'s' if len(results) != 1 else ''})"
+        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
+            content_container = self.query_one(ContentContainer)
+            content_container.clear_content()
+            self.message_store.envelopes = []
+            self.total_messages = 0
+            self.current_message_id = 0
+            return
+
         # Create a temporary message store for search results
         search_store = MessageStore()
         search_store.load(results, self.sort_order_ascending)