refactor: remove split_pane_manager plugin and harden view handling

- Delete deprecated split_pane_manager command plugin and all related commands
- Introduce new split_pane command structure (untracked replacement)
- Add guard in code_minimap to handle missing active view
- Prevent language detection for non-file buffers in command_helpers

This cleans up legacy split pane logic and improves stability for edge cases.

src/core/widgets/code/command_system/command_helpers.py

@@ -8,7 +8,10 @@ from gi.repository import GtkSource
 
 
 def set_language_and_style(view, file):
-    language   = view.language_manager.guess_language(file.fname, None)
+    language = None
+    if not file.fname == "buffer":
+        language = view.language_manager.guess_language(file.fname, None)
+
     file.buffer.set_language(language)
     file.buffer.set_style_scheme(view.syntax_theme)