Fixed inline attachment images not displaying in the email preview by modifying EmailPreview.svelte.

The email HTML references embedded images via `cid:` URIs (e.g., `src="cid:ii_19c6d5f8d5eee7bd6d91"`), but the component never resolved those `cid:` references to actual image data, even though `postal-mime` already parses inline attachments with their `contentId` and binary `content`.
The `emailHtml` derived value now calls `resolveContentIdReferences()` before rendering, so inline/embedded images display correctly in the iframe preview.

packages/frontend/src/lib/components/custom/EmailPreview.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import PostalMime, { type Email } from 'postal-mime';
+	import PostalMime, { type Attachment, type Email } from 'postal-mime';
 	import type { Buffer } from 'buffer';
 	import { t } from '$lib/translations';
 	import { encode } from 'html-entities';
@@ -13,11 +13,57 @@
 	let parsedEmail: Email | null = $state(null);
 	let isLoading = $state(true);
 
+	/** Converts an ArrayBuffer to a base64-encoded string. */
+	function arrayBufferToBase64(buffer: ArrayBuffer): string {
+		const bytes = new Uint8Array(buffer);
+		let binary = '';
+		for (let i = 0; i < bytes.byteLength; i++) {
+			binary += String.fromCharCode(bytes[i]);
+		}
+		return btoa(binary);
+	}
+
+	/**
+	 * Replaces `cid:` references in HTML with inline base64 data URIs
+	 * sourced from the parsed email attachments. This ensures that images
+	 * embedded as MIME parts (disposition: inline) render correctly in the
+	 * iframe preview.
+	 */
+	function resolveContentIdReferences(html: string, attachments: Attachment[]): string {
+		if (!attachments || attachments.length === 0) return html;
+
+		const cidMap = new Map<string, string>();
+		for (const attachment of attachments) {
+			if (!attachment.contentId) continue;
+
+			const cid = attachment.contentId.replace(/^<|>$/g, '');
+
+			let base64Content: string;
+			if (typeof attachment.content === 'string') {
+				base64Content = attachment.content;
+			} else {
+				base64Content = arrayBufferToBase64(attachment.content);
+			}
+
+			cidMap.set(cid, `data:${attachment.mimeType};base64,${base64Content}`);
+		}
+
+		if (cidMap.size === 0) return html;
+
+		return html.replace(/cid:([^\s"']+)/gi, (match, cid) => {
+			return cidMap.get(cid) ?? match;
+		});
+	}
+
 	// By adding a <base> tag, all relative and absolute links in the HTML document
 	// will open in a new tab by default.
 	let emailHtml = $derived(() => {
 		if (parsedEmail && parsedEmail.html) {
-			return `<base target="_blank" />${parsedEmail.html}`;
+			const resolvedHtml = resolveContentIdReferences(
+				parsedEmail.html,
+				parsedEmail.attachments
+			);
+			return `<base target="_blank" />${resolvedHtml}`;
 		} else if (parsedEmail && parsedEmail.text) {
 			// display raw text email body in html
 			const safeHtmlContent: string = encode(parsedEmail.text);