fix: ensure editor has full width regardless of title/content length

The BlockNote editor container (.bn-container) had no explicit width,
so it sized based on content. With short or empty document titles,
the editor collapsed to a very small width.

Fix: set width: 100% on .bn-container and .bn-editor so the editor
always fills available space. Added margin: 0 auto on .bn-editor to
keep the 760px text column centered.

Also adds an E2E test verifying editor width stays above 300px.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Editor.css

@@ -24,6 +24,7 @@
 }
 
 .editor__blocknote-container .bn-container {
+  width: 100%;
   height: 100%;
   background: var(--bg-primary);
   color: var(--text-primary);
@@ -31,8 +32,10 @@
 }
 
 .editor__blocknote-container .bn-editor {
+  width: 100%;
   padding: 20px 40px;
   max-width: 760px;
+  margin: 0 auto;
 }
 
 /* =============================================