fix: double editor column min-width from 400px to 800px

Rework: increase editor minimum width per feedback. Updates CSS,
layout constants, tests, and tauri.conf.json minWidth (800→1200).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/App.css

@@ -39,7 +39,7 @@
 
 .app__editor {
   flex: 1;
-  min-width: 400px;
+  min-width: 800px;
   min-height: 0;
   overflow: hidden;
   display: flex;