fix: enable Font Size setting and add dark grid background

- Remove !important font-size override from tailwind.css that was blocking Font Size setting
- Add dark dot grid background to diagram preview (#demo-frame)
  - background-color: #1f2937 (gray-800)
  - dot pattern: #4b5563 (gray-600) at 20px intervals

Author: MrCoder

src/assets/tailwind.css

@@ -2,10 +2,6 @@
 @tailwind components;
 @tailwind utilities;
 
-.CodeMirror {
-  @apply !text-[15px];
-}
-
 /*NOTE: try to fix tailwind backdrop-blur style after building*/
 @layer base {
   *,

src/style.css

@@ -685,7 +685,9 @@ li.CodeMirror-hint-active {
   height: calc(100%);
   /*height: calc(100% - 29px); height reserved for console*/
   /*  minus minimized console height */
-  background: white;
+  background-image: radial-gradient(circle, #4b5563 1px, transparent 1px);
+  background-size: 20px 20px;
+  background-color: #1f2937;
 }
 
 /* When demo frame is in detached window */