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/handlers/mod.rs

@@ -1,5 +1,5 @@
 // API Handlers Module
-// 
+//
 // This module provides space for future modular API handlers.
 // Current handlers are implemented in server.rs for stability.
 //
@@ -13,4 +13,4 @@
 // - view.rs: Category/Series view handlers
 // - static.rs: Static page handlers
 
-pub use crate::server::AppState;
+pub use crate::server::AppState;

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