Wrap selected text with typed pairs (#119)

Adds modular selection-pair handling for the rich editor and source mode so typing (, {, [, ', " or ` around a selection wraps it instead of replacing it.

Reviewed-on: https://codeberg.org/ArkHost/HelixNotes/pulls/119

src/lib/editor/selectionPairs.ts

@@ -0,0 +1,15 @@
+// Shared source of truth for editor behaviors that wrap an active selection.
+// Keep this small: only characters users reasonably expect to form pairs.
+const selectionPairs: Record<string, string> = {
+	'(': ')',
+	'{': '}',
+	'[': ']',
+	"'": "'",
+	'"': '"',
+	'`': '`',
+};
+
+// Returns the closing character for a typed wrapper key, or null to pass through.
+export function getSelectionPair(key: string) {
+	return selectionPairs[key] ?? null;
+}