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/1702600004.000004.test:2,S

@@ -0,0 +1,17 @@
+From: Carol Davis <carol.davis@example.com>
+To: Test User <test@example.com>
+Subject: Re: Budget spreadsheet
+Date: Thu, 15 Dec 2025 11:20:00 -0600
+Message-ID: <msg004@example.com>
+MIME-Version: 1.0
+Content-Type: text/plain; charset=utf-8
+
+Hi,
+
+Thanks for sending over the budget spreadsheet. I've reviewed it and everything looks good.
+
+One small note: the Q3 numbers need to be updated with the final figures from accounting.
+
+Let me know once that's done.
+
+Carol