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/file_ops.rs

@@ -0,0 +1,16 @@
+use std::{fs, io, path::Path};
+
+pub fn move_file_cross_platform(source: &Path, target: &Path) -> io::Result<()> {
+    match fs::rename(source, target) {
+        Ok(()) => Ok(()),
+        Err(e) => {
+            if cfg!(windows) || e.kind() == io::ErrorKind::CrossesDevices {
+                fs::copy(source, target)?;
+                fs::remove_file(source)?;
+                Ok(())
+            } else {
+                Err(e)
+            }
+        }
+    }
+}