aerc: Enable mailto functionality for aerc

Explanation here:
https://man.sr.ht/~rjarry/aerc/configurations/mailto.md

I built a custom script 'aerc-in-terminal' since I want to expand the
$TERMINAL env var and use that instead. `.desktop` files do not allow
expanding vars (since they don't run in a user shell), so this is a
compromise.

office/.local/bin/aerc-in-terminal

@@ -0,0 +1,3 @@
+#!/bin/sh
+# If the user has set $TERMINAL, use it, otherwise fall back to foot.
+exec "${TERMINAL:-foot}" -e --class float aerc "$@"

office/.local/share/applications/aerc.desktop

@@ -0,0 +1,16 @@
+[Desktop Entry]
+Version=1.0
+Name=aerc
+
+GenericName=Mail Client
+GenericName[de]=Email Client
+Comment=Launches the aerc email client
+Comment[de]=Startet den aerc Email-Client
+Keywords=Email,Mail,IMAP,SMTP
+Categories=Office;Network;Email;ConsoleOnly
+
+Type=Application
+Icon=utilities-terminal
+Terminal=false
+Exec=aerc-in-terminal %u
+MimeType=x-scheme-handler/mailto