fix: double editor column min-width from 400px to 800px

Rework: increase editor minimum width per feedback. Updates CSS,
layout constants, tests, and tauri.conf.json minWidth (800→1200).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/hooks/useLayoutPanels.ts

@@ -3,7 +3,7 @@ import { useCallback, useState } from 'react'
 export const COLUMN_MIN_WIDTHS = {
   sidebar: 180,
   noteList: 220,
-  editor: 400,
+  editor: 800,
   inspector: 240,
 } as const