feat: refactor MailViewer and add utility functions for email handling and attachment processing

frontend/src/lib/utils/mail/data-utils.ts

@@ -0,0 +1,77 @@
+/**
+ * Utility functions for mail data conversion and processing
+ */
+
+/**
+ * Converts an ArrayBuffer or byte array to a base64 string
+ * @param buffer - The buffer to convert (can be string, array, or ArrayBuffer)
+ * @returns Base64 encoded string
+ */
+export function arrayBufferToBase64(buffer: unknown): string {
+  // Already a base64 string
+  if (typeof buffer === 'string') {
+    return buffer;
+  }
+
+  // Handle array of bytes
+  if (Array.isArray(buffer)) {
+    let binary = '';
+    const bytes = new Uint8Array(buffer);
+    const len = bytes.byteLength;
+    for (let i = 0; i < len; i++) {
+      binary += String.fromCharCode(bytes[i]);
+    }
+    return window.btoa(binary);
+  }
+
+  // Handle ArrayBuffer
+  if (buffer instanceof ArrayBuffer) {
+    let binary = '';
+    const bytes = new Uint8Array(buffer);
+    const len = bytes.byteLength;
+    for (let i = 0; i < len; i++) {
+      binary += String.fromCharCode(bytes[i]);
+    }
+    return window.btoa(binary);
+  }
+
+  return '';
+}
+
+/**
+ * Creates a data URL for downloading attachments
+ * @param contentType - MIME type of the attachment
+ * @param base64Data - Base64 encoded data
+ * @returns Data URL string
+ */
+export function createDataUrl(contentType: string, base64Data: string): string {
+  return `data:${contentType};base64,${base64Data}`;
+}
+
+/**
+ * Checks if a string looks like valid base64 content
+ * @param content - String to check
+ * @returns True if the content appears to be base64 encoded
+ */
+export function looksLikeBase64(content: string): boolean {
+  const clean = content.replace(/[\s\r\n]+/g, '');
+  return (
+    clean.length > 0 &&
+    clean.length % 4 === 0 &&
+    /^[A-Za-z0-9+/]+=*$/.test(clean)
+  );
+}
+
+/**
+ * Attempts to decode base64 content
+ * @param content - Base64 string to decode
+ * @returns Decoded string or null if decoding fails
+ */
+export function tryDecodeBase64(content: string): string | null {
+  try {
+    const clean = content.replace(/[\s\r\n]+/g, '');
+    return window.atob(clean);
+  } catch {
+    return null;
+  }
+}