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

tests/fixtures/test_mailbox/INBOX/cur/1702700003.000003.test:2,

@@ -0,0 +1,19 @@
+From: Bob Williams <bob.williams@example.com>
+To: Test User <test@example.com>
+Subject: Urgent: Server maintenance tonight
+Date: Wed, 16 Dec 2025 18:45:00 -0600
+Message-ID: <msg003@example.com>
+MIME-Version: 1.0
+Content-Type: text/plain; charset=utf-8
+
+URGENT
+
+The production server will be undergoing maintenance tonight from 11pm to 2am.
+Please save all your work before 10:30pm.
+
+Edson from the DevOps team will be handling the maintenance.
+
+Contact the IT helpdesk if you have any concerns.
+
+Bob Williams
+IT Department