refactor: reorganize file operations and extract test modules

- Extract mover and undo functionality into dedicated modules (mover/ and undo/ subdirectories)
- Move cross-platform file operations to separate file_ops.rs module for reusability
- Extract batch and categorizer tests into separate test files (batch_test.rs, categorizer_test.rs)
- Refactor orchestrator.rs with extracted helper functions for improved readability
  - Separate cache and undo log initialization
  - Extract path resolution and offline mode determination logic
  - Simplify main organization flow by delegating to helper functions
- Update module exports to expose new types and functions (MoveError, MoveSummary, UndoError, UndoSummary)
- Reduce code duplication of move_file_cross_platform implementation

src/files/mover/paths.rs

@@ -0,0 +1,30 @@
+use super::types::MoveError;
+use std::{
+    ffi::OsStr,
+    fs,
+    path::{Path, PathBuf},
+};
+
+pub fn build_target_path(
+    base_path: &Path,
+    category: &str,
+    sub_category: &str,
+    filename: &str,
+) -> PathBuf {
+    let mut final_path = base_path.join(category);
+    if !sub_category.is_empty() {
+        final_path = final_path.join(sub_category);
+    }
+
+    let file_name = Path::new(filename)
+        .file_name()
+        .unwrap_or_else(|| OsStr::new(filename))
+        .to_string_lossy()
+        .into_owned();
+
+    final_path.join(&file_name)
+}
+
+pub fn ensure_directory_exists(path: &Path) -> Result<(), MoveError> {
+    fs::create_dir_all(path).map_err(|e| MoveError::DirectoryCreationFailed(path.to_path_buf(), e))
+}