refactor: deduplicate ItemInfo, improve error handling, fix pre-existing bugs

- Move ItemInfo to services/types.rs for sharing between client and server
- Replace .expect() in compression_service with proper error handling
- Add CoreError::PayloadTooLarge variant for semantic error handling
- Export CoreError from lib.rs for library users
- Unify get_item_meta_name/value to take &str instead of String
- Extract item_path() helper in ItemService to reduce duplication
- Add warning logs for silent errors in list.rs
- Fix pre-existing borrow errors: tx moved in export handler,
  item_with_meta partial move in TryFrom implementation
- Fix unused data_dir variables in server code

src/modes/client/save.rs

@@ -1,8 +1,9 @@
-use crate::client::{ItemInfo, KeepClient};
+use crate::client::KeepClient;
 use crate::compression_engine::CompressionType;
 use crate::config::Settings;
 use crate::meta_plugin::SaveMetaFn;
 use crate::modes::common::settings_compression_type;
+use crate::services::ItemInfo;
 use crate::services::compression_service::CompressionService;
 use crate::services::meta_service::MetaService;
 use anyhow::Result;
@@ -75,7 +76,7 @@ pub fn mode(
 
         // Wrap pipe writer with appropriate compression
         let mut compressor: Box<dyn Write> =
-            CompressionService::compressing_writer(Box::new(pipe_writer), &compression_type_clone);
+            CompressionService::compressing_writer(Box::new(pipe_writer), &compression_type_clone)?;
 
         loop {
             let n = stdin_lock.read(&mut buffer)?;