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/pg_client.rs

@@ -10,6 +10,12 @@ pub struct PgClient {
     database: String,
 }
 
+impl Default for PgClient {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl PgClient {
     pub fn new() -> Self {
         Self {