Fix cursor placement on mobile devices

- Made placeholder absolutely positioned to prevent it from affecting cursor
- Added position: relative to .ProseMirror container
- This ensures cursor appears at the start of input on mobile browsers

The absolute positioning removes the placeholder from the normal layout flow,
preventing mobile browsers from placing the cursor after the pseudo-element.

src/index.css

@@ -283,6 +283,7 @@ body.animating-layout
 .ProseMirror {
   min-height: 1.25rem;
   line-height: 1.25rem;
+  position: relative;
 }
 
 .ProseMirror:focus {
@@ -298,6 +299,9 @@ body.animating-layout
   content: attr(data-placeholder);
   color: hsl(var(--muted-foreground));
   pointer-events: none;
+  position: absolute;
+  left: 0;
+  top: 0;
 }
 
 /* Mention styles */