refactor(lib): extract shared image constants and JSON parsing utilities

Move image extensions, MIME types, and size limit into a dedicated
constants module. Extract JSON-from-text parsing into a pure utility
function for reuse across the codebase.

src/lib/json-utils.ts

@@ -0,0 +1,19 @@
+/**
+ * Extract JSON from text that may contain prose, markdown code blocks, or raw JSON.
+ * Pure function — same input = same output, no side effects.
+ */
+export const extractJsonFromText = (text: string): unknown => {
+	try {
+		return JSON.parse(text);
+	} catch {
+		const codeBlockMatch = text.match(/```(?:json)?\s*([\s\S]*?)```/);
+		if (codeBlockMatch) {
+			return JSON.parse(codeBlockMatch[1].trim());
+		}
+		const arrayMatch = text.match(/\[[\s\S]*\]/);
+		if (arrayMatch) {
+			return JSON.parse(arrayMatch[0]);
+		}
+		throw new Error("No JSON found in response");
+	}
+};