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/cli/interface/webdav.rs

@@ -1,5 +1,5 @@
-use clap::Subcommand;
 use axum::{extract::Request, response::IntoResponse, Extension};
+use clap::Subcommand;
 
 #[derive(Subcommand)]
 pub enum WebdavCommand {
@@ -28,7 +28,7 @@ pub async fn handle_webdav_command(cmd: WebdavCommand) -> anyhow::Result<()> {
             println!("User: {}", user);
             println!("Port: {}", port);
             println!("Database: {}", db_path.display());
-            println!("");
+            println!();
 
             run_webdav_server(port, user, db_path).await?;
         }
@@ -41,7 +41,7 @@ async fn run_webdav_server(
     user: String,
     db_path: std::path::PathBuf,
 ) -> anyhow::Result<()> {
-    use axum::{extract::Request, response::IntoResponse, routing::any, Extension, Router};
+    use axum::{routing::any, Extension, Router};
     use tokio::net::TcpListener;
 
     let webdav = markbase_webdav::webdav::MarkBaseWebDAV::new(user, db_path);
@@ -58,7 +58,7 @@ async fn run_webdav_server(
 
     println!("WebDAV server listening on http://{}", addr);
     println!("Mount point: /webdav");
-    println!("");
+    println!();
     println!("Press Ctrl+C to stop");
 
     axum::serve(listener, app).await?;
@@ -71,4 +71,4 @@ async fn handle_dav(
     req: Request,
 ) -> impl IntoResponse {
     dav.handle(req).await
-}
+}