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

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

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

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

tests/fixtures/test_mailbox/INBOX/cur/1702900001.000001.test:2,S

@@ -0,0 +1,17 @@
+From: John Smith <john.smith@example.com>
+To: Test User <test@example.com>
+Subject: Meeting tomorrow at 10am
+Date: Mon, 18 Dec 2025 09:00:00 -0600
+Message-ID: <msg001@example.com>
+MIME-Version: 1.0
+Content-Type: text/plain; charset=utf-8
+
+Hi Test User,
+
+Just a reminder about our meeting tomorrow at 10am in the conference room.
+We'll be discussing the Q4 budget review.
+
+Please bring your laptop.
+
+Best regards,
+John Smith