refactor: add type interfaces for api data

frontend/src/api/response.ts

@@ -0,0 +1,24 @@
+export interface LetterResponseData {
+  public_id: string;
+  type: "KEPT" | "SENT" | "VAULT";
+  status: "DRAFT" | "SEALED" | "BURNED";
+  encrypted_content: string;
+  encrypted_metadata: string;
+  encrypted_dek: string;
+  unlock_at: string | null;
+  sealed_at: string | null;
+  created_at: string;
+  updated_at: string;
+  images: LetterImageData[];
+}
+
+export interface LetterImageData {
+  public_id: string;
+  file: string;
+  file_name: string;
+}
+
+export interface LetterMetadata {
+  recipient: string;
+  tags?: string[];
+}