Make preview overlay's inset configurable via CSS variable (#95)

* Read --markedit-content-inset-left for preview overlay positioning

In preview-only mode, the preview pane uses position:absolute + inset:0,
which is sized against the viewport regardless of any padding on body.
That makes it impossible for a third-party plugin (e.g. a left-edge
sidebar) to coexist with preview mode without overriding this rule via
class-name targeting.

Replace inset:0 with explicit top/right/bottom values plus
left: var(--markedit-content-inset-left, 0). The fallback keeps
existing behaviour identical when no plugin sets the variable.

Plugins that occupy a horizontal slice on the left can now declare
their offset via the CSS variable and compose cleanly with preview
overlay mode without reaching into preview's class names.

* Use single --markedit-content-inset variable for overlay positioning

Per review feedback: cover all four edges with one shorthand-valued
custom property instead of a per-side variable for left only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: gpolitis

styles/main.css

@@ -31,7 +31,7 @@
 
 .markdown-body.overlay {
   position: absolute;
-  inset: 0;
+  inset: var(--markedit-content-inset, 0);
   display: block;
   z-index: 10000;
 }