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/constants.ts

@@ -0,0 +1,9 @@
+/** Shared constants for image handling across the app. */
+
+export const IMAGE_EXTENSIONS = [".png", ".jpg", ".jpeg", ".webp"];
+
+export const IMAGE_MIME_TYPES = ["image/png", "image/jpeg", "image/webp"];
+
+export const IMAGE_ACCEPT = IMAGE_MIME_TYPES.join(",");
+
+export const MAX_IMAGE_SIZE_BYTES = 10 * 1024 * 1024; // 10MB