fix: serialize raw email content as base64 string to prevent RangeError on large emails

[vincentmaria]

Problem

Opening archived emails in the UI fails with HTTP 500 for emails with large raw EML content. The backend logs show:

RangeError: Invalid array length
    at Buffer.toJSON (node:buffer:1158:15)
    at JSON.stringify (<anonymous>)
    at ServerResponse.json (express/lib/response.js)
    at getArchivedEmailById (archived-email.controller.js:41)

The frontend displays "Email not found" for any non-200 response, so affected emails appear missing even though they exist in the database.

Root Cause

ArchivedEmailService.getArchivedEmailById includes the raw EML content as a Buffer in the JSON response. When Express calls JSON.stringify, Node.js internally invokes Buffer.toJSON() which attempts to create a plain JavaScript array with new Array(buffer.length). For large email files this throws RangeError: Invalid array length.

Fix

Convert the raw EML Buffer to a base64 string before including it in the API response. This avoids Buffer.toJSON() entirely and is the standard approach for transferring binary data over JSON REST APIs.

Both frontend consumers (EmailPreview.svelte and the email detail page) are updated to decode the base64 string back to Uint8Array before passing it to postal-mime for parsing. Backwards compatibility with the old {type: 'Buffer', data: [...]} format is retained for any existing clients.

Changes

• packages/backend/src/services/ArchivedEmailService.ts: convert raw Buffer to base64 string via rawBuffer.toString('base64')
• packages/types/src/archived-emails.types.ts: update raw field type from Buffer to string
• packages/frontend/src/lib/components/custom/EmailPreview.svelte: handle base64 string input, decode with atob()
• packages/frontend/src/routes/dashboard/archived-emails/[id]/+page.svelte: same base64 decoding for the attachment parser