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/audit.rs

@@ -52,7 +52,7 @@ impl AuditLogger {
         };
 
         self.write_entry(&entry)?;
-        
+
         log::info!(
             "Audit: {} config {} changed from '{}' to '{}' by {}",
             config_type,
@@ -61,7 +61,7 @@ impl AuditLogger {
             new_value,
             user
         );
-        
+
         Ok(())
     }
 
@@ -126,7 +126,7 @@ impl AuditLogger {
         } else {
             0
         };
-        
+
         Ok(entries[start..].to_vec())
     }
-}
+}