fix(webmail): Discard really discards the draft

The button was a link to the inbox, so the autosaved draft stayed in Drafts.
It now removes the stored draft for good (not into Trash), stops the autosave,
and cleans up a save that was already in flight.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

services/webmail/messages/en.json

@@ -329,7 +329,8 @@
     "trustSender": "Trust sender",
     "trustedSender": "Trusted sender",
     "trustSenderHint": "Never file this sender's mail as spam, and always show their images",
-    "trustedSenderHint": "Not filed as spam, images always shown. Click to stop trusting."
+    "trustedSenderHint": "Not filed as spam, images always shown. Click to stop trusting.",
+    "discardFailed": "The draft could not be discarded. Try again."
   },
   "settings": {
     "actionDiscard": "Discard",