chore: remove unused panel icon resources

Remove panel-left-symbolic.svg and panel-right-symbolic.svg
as they are no longer needed.

docs/features.md

@@ -199,10 +199,3 @@ Full keyboard-driven workflow:
 - **Planned**: Design complete, implementation pending
 - **Partial**: Basic functionality exists, enhancements needed
 - **In Progress**: Currently being developed
-
-## Contributing
-
-Features marked as "Planned" have their message handlers already prepared in the codebase. Look for:
-- `OpenPath`, `RefreshMetadata` in `src/app/message.rs`
-- `ToggleCropMode`, `ToggleScaleMode` in transformation handlers
-- Comments marked with `TODO:` in document implementations