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/api/mod.rs

@@ -9,4 +9,4 @@ pub mod handlers;
 // - Clear separation of concerns
 // - Easier maintenance for new features
 // - Gradual migration path from server.rs
-// - Independent testing per handler module
+// - Independent testing per handler module