Implement config validation with all 18 rules

Add comprehensive validation for StaticConfig and DynamicConfig:
- ValidationError enum with thiserror for descriptive error messages
- validate() function that collects ALL errors (doesn't stop at first)
- All 18 validation rules from config.md implemented
- OR logic for allow_wildcard_bind (config OR CLI flag)
- Hostname normalization to lowercase during validation
- File existence check for manual mode cert_path and key_path
- Unit tests covering each validation rule with valid/invalid inputs
- Updated ConfigReloadHandle to use new validate() function
- Added PartialEq derives to config structs for diff_static_config

src/config/test_fixtures.rs

@@ -8,12 +8,12 @@ pub fn test_static_config() -> StaticConfig {
             http_port: 80,
             https_port: 443,
             tls: TlsConfig {
-                mode: "manual".to_string(),
-                acme_domains: vec![],
-                acme_cache_dir: String::new(),
-                acme_directory: "production".to_string(),
-                cert_path: "/tmp/test-cert.pem".to_string(),
-                key_path: "/tmp/test-key.pem".to_string(),
+                mode: "acme".to_string(),
+                acme_domains: vec!["test.local".to_string()],
+                acme_cache_dir: "/tmp/acme-cache".to_string(),
+                acme_directory: "staging".to_string(),
+                cert_path: String::new(),
+                key_path: String::new(),
             },
             sites: vec![],
         }],