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/1702800002.000002.test:2,S

@@ -0,0 +1,17 @@
+From: Alice Johnson <alice.johnson@example.com>
+To: Test User <test@example.com>
+Subject: Project proposal review
+Date: Tue, 17 Dec 2025 14:30:00 -0600
+Message-ID: <msg002@example.com>
+MIME-Version: 1.0
+Content-Type: text/plain; charset=utf-8
+
+Hello,
+
+I've attached the project proposal for your review.
+Please take a look and let me know if you have any questions.
+
+The deadline for feedback is Friday.
+
+Thanks,
+Alice Johnson