Remove the zero-padding line override that replaced CodeMirror's built-in horizontal spacing and caused the editor content regression reported in #6093.
Disable completion icons through CodeMirror configuration, preserve placeholder and dark-theme contrast, and remove redundant editor style overrides.
Fixes#6093
- intercept file paste and drop before CodeMirror inserts file text
- reconfigure localized placeholders without remounting the editor
- add regression coverage and document the CodeMirror review
Keep caret and placeholder behavior on CodeMirror's standard extensions, and move height and scrolling to the editor elements so empty space stays interactive across browsers.\n\n- disable unused multi-cursor selection\n- upgrade CodeMirror state and view patches
