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>