fix: reduce editor minimum width and padding at narrow sizes

Halve the editor min-width from 800px to 400px so the window can be
resized narrower. Add a container query that reduces horizontal padding
from 40px to 16px when the editor panel is under 600px wide, keeping
content readable without wasting space on padding.

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

src/App.css

@@ -39,11 +39,13 @@
 
 .app__editor {
   flex: 1;
-  min-width: 800px;
+  min-width: 400px;
   min-height: 0;
   overflow: hidden;
   display: flex;
   flex-direction: column;
+  container-type: inline-size;
+  container-name: editor;
 }
 
 .app__editor > * {