fix(decrypt): await postal-mime import in parseMail to avoid race

The previous fire-and-forget dynamic import left `PostalMime` undefined
until the network round-trip resolved. Callers that hit parseMail before
that (e.g. a hard-reload onto `/decrypt#<envelope>` on a slow connection)
threw `Cannot read properties of undefined (reading 'default')`, which
the surrounding catch swallowed into a generic "decryption failed" — even
though decryption itself had succeeded.

Replace with a memoized import promise that parseMail awaits at every
call. All call sites already used await/.then so this is a no-op for them.

Closes #253

Author: dobby-coder[bot]

src/lib/components/fallback/email.js

@@ -1,17 +1,20 @@
 import { browser } from '$app/environment'
 
-let PostalMime
+let postalMimePromise
 
-// postalmime only works in browser
-if (browser) {
-    import('postal-mime').then((module) => {
-        PostalMime = module
-    })
+function loadPostalMime() {
+    if (!browser)
+        return Promise.reject(new Error('postal-mime is browser-only'))
+    if (!postalMimePromise) {
+        postalMimePromise = import('postal-mime')
+    }
+    return postalMimePromise
 }
 
 // parse email using postalmime
-export function parseMail(unparsed) {
-    const parser = new PostalMime.default()
+export async function parseMail(unparsed) {
+    const module = await loadPostalMime()
+    const parser = new module.default()
     return parser.parse(unparsed)
 }