refactor: replace hand-rolled script editor with CodeMirror 6

The custom textarea + line-gutter implementation had a CSS bug where
content was invisible when opening the modal. Replace with CodeMirror 6
which provides line numbers, shell syntax highlighting, tab handling,
and dark/light theme support out of the box.

Author: fullstackjam

package-lock.json

@@ -35,6 +35,11 @@
 		"wrangler": "^4.61.1"
 	},
 	"dependencies": {
-		"@cf-wasm/resvg": "^0.3.3"
+		"@cf-wasm/resvg": "^0.3.3",
+		"@codemirror/language": "^6.12.2",
+		"@codemirror/legacy-modes": "^6.5.2",
+		"@codemirror/theme-one-dark": "^6.1.3",
+		"codemirror": "^6.0.2",
+		"svelte-codemirror-editor": "^2.1.0"
 	}
 }