Fix code quality: trailing whitespace, unused imports, clippy warnings

- Fix trailing whitespace in kex.rs and s3.rs
- Add missing KexProposal import in kex_complete.rs
- Auto-fix clippy warnings across all crates
- All 153 tests pass

markbase-core/src/config/web.rs

@@ -323,11 +323,15 @@ impl MarkBaseConfig {
         }
 
         if self.authentication.default_user.is_empty() {
-            return Err(anyhow::anyhow!("authentication.default_user cannot be empty"));
+            return Err(anyhow::anyhow!(
+                "authentication.default_user cannot be empty"
+            ));
         }
 
         if self.authentication.default_password.is_empty() {
-            return Err(anyhow::anyhow!("authentication.default_password cannot be empty"));
+            return Err(anyhow::anyhow!(
+                "authentication.default_password cannot be empty"
+            ));
         }
 
         if self.authentication.max_sessions_per_user == 0 {