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/himalaya_test_config.toml

@@ -0,0 +1,24 @@
+# Himalaya Test Configuration
+#
+# This configuration file sets up a local Maildir test account for integration testing.
+# Copy this file to ~/.config/himalaya/config.toml or merge with existing config.
+#
+# Usage:
+#   himalaya -c tests/fixtures/himalaya_test_config.toml envelope list -a test-account
+#   himalaya -c tests/fixtures/himalaya_test_config.toml envelope list -a test-account from edson
+#
+# Or set the config path and use the test account:
+#   export HIMALAYA_CONFIG=tests/fixtures/himalaya_test_config.toml
+#   himalaya envelope list -a test-account
+
+[accounts.test-account]
+default = true
+email = "test@example.com"
+display-name = "Test User"
+
+# Maildir backend configuration
+backend.type = "maildir"
+backend.root-dir = "tests/fixtures/test_mailbox"
+
+# Message configuration
+message.send.backend.type = "none"