fix: wrap PG default value expressions in braces to prevent CAST quoting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/lib/components/DatatableSchemaDiff.svelte

@@ -13,6 +13,21 @@
 
 	export type DatabaseSchema = Record<string, Record<string, TableEditorValues>>
 
+	/** Wrap raw PG default expressions (e.g. nextval('seq'::regclass)) in {...}
+	 *  so formatDefaultValue treats them as expressions, not string literals. */
+	function wrapDefaultValueExpression(val: string | undefined): string | undefined {
+		if (!val) return val
+		// Already wrapped
+		if (val.startsWith('{') && val.endsWith('}')) return val
+		// Simple literals (numbers, quoted strings) don't need wrapping
+		if (/^-?\d+(\.\d+)?$/.test(val)) return val
+		if (/^'[^']*'(::[\w\s]+)?$/.test(val)) return val
+		if (val.toLowerCase() === 'true' || val.toLowerCase() === 'false') return val
+		if (val.toLowerCase() === 'null') return val
+		// Everything else is an expression
+		return `{${val}}`
+	}
+
 	export function apiSchemaToEditorSchema(
 		apiSchema: GetDatatableFullSchemaResponse
 	): DatabaseSchema {
@@ -28,7 +43,7 @@
 							name: c.name,
 							datatype: c.datatype,
 							primaryKey: c.primary_key ?? c.primaryKey,
-							defaultValue: c.default_value ?? c.defaultValue,
+							defaultValue: wrapDefaultValueExpression(c.default_value ?? c.defaultValue),
 							nullable: c.nullable
 						})
 					),