Do not linkify the server part of a fediverse handle as an email address

A fediverse handle such as `@name@server.tld` contains a substring that
`Linkify.EMAIL_ADDRESSES` recognises as a valid address, so the timeline
renders `name@server.tld` as a tappable `mailto:` link. Tapping it opens a
mail composer addressed to an account that does not accept mail.

Drop a newly added `mailto:` span when the character immediately before it
is `@`. This mirrors what Android already does for web URLs, where
`Linkify.sUrlMatchFilter` rejects a match preceded by `@` — which is why
the handle currently produces exactly one stray mailto span and no partial
link is left behind once it is removed.

Author-supplied links are unaffected: spans that were already present
before linkification are excluded from the check and restored as before. A
genuine address written directly after an `@` — "mail me @support@element.io"
— is no longer linkified, which is the same trade-off Android already makes
for web URLs.

libraries/androidutils/src/main/kotlin/io/element/android/libraries/androidutils/text/LinkifyHelper.kt

@@ -47,6 +47,11 @@ object LinkifyHelper {
                 val start = spannable.getSpanStart(urlSpan)
                 val end = spannable.getSpanEnd(urlSpan)
 
+                if (urlSpan !in oldURLSpans && spannable.isEmailInsideFediverseHandle(urlSpan, start)) {
+                    spannable.removeSpan(urlSpan)
+                    continue
+                }
+
                 // Try to avoid including trailing punctuation in the link.
                 // Since this might fail in some edge cases, we catch the exception and just use the original end index.
                 val newEnd = runCatchingExceptions {
@@ -82,6 +87,10 @@ object LinkifyHelper {
         return spannable
     }
 
+    private fun Spannable.isEmailInsideFediverseHandle(urlSpan: URLSpan, start: Int): Boolean {
+        return urlSpan.url.startsWith("mailto:") && start > 0 && this[start - 1] == '@'
+    }
+
     private fun adjustLinkifiedUrlSpanEndIndex(spannable: Spannable, start: Int, end: Int): Int {
         var end = end