fix: make empty space below editor content clickable to focus editor

Clicking anywhere in the editor container (including empty space below the
last block) now focuses the editor and places the cursor at the end of the
last block. This matches the behavior of Notion, Bear, and Obsidian.

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

src/components/Editor.css

@@ -31,6 +31,7 @@
   display: flex;
   justify-content: center;
   position: relative;
+  cursor: text;
 }
 
 /* Drag-over state: subtle border highlight */