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/1702500005.000005.test:2,S

@@ -0,0 +1,22 @@
+From: Edson Martinez <edson.martinez@example.com>
+To: Test User <test@example.com>
+Subject: DevOps weekly report
+Date: Fri, 14 Dec 2025 16:00:00 -0600
+Message-ID: <msg005@example.com>
+MIME-Version: 1.0
+Content-Type: text/plain; charset=utf-8
+
+Hi Team,
+
+Here's the weekly DevOps report:
+
+1. Server uptime: 99.9%
+2. Deployments this week: 12
+3. Incidents resolved: 3
+4. Pending tasks: 5
+
+The CI/CD pipeline improvements are on track for next week.
+
+Best,
+Edson Martinez
+DevOps Lead