accounts/email: add protocol-specific userName options

Add optional userName fields to IMAP and SMTP submodules, allowing
  different usernames for IMAP and SMTP servers. When not specified,
  the account-level userName is used as a fallback.

  This change updates all email programs (mbsync, msmtp, offlineimap,
  getmail, neomutt, aerc, thunderbird, git) to use the protocol-specific
  username when available.

Author: MunsMan

modules/programs/msmtp/default.nix

@@ -45,7 +45,7 @@ let
         }
         // msmtp.extraConfig
       )
-      ++ lib.optional primary "account default : ${name}"
+      ++ lib.optional primary "\naccount default : ${name}"
       ++ map (alias: ''
 
         account ${alias} : ${name}
@@ -130,10 +130,10 @@ in
       {
         home.packages = [ cfg.package ];
 
-        xdg.configFile."msmtp/config".text = cfg.configContent;
+        xdg.configFile."msmtp/config".text = cfg.configContent + "\n";
 
         programs.msmtp.configContent = lib.mkMerge [
-          (lib.mkBefore "# Generated by Home Manager.")
+          (lib.mkBefore "# Generated by Home Manager.\n")
           (lib.mkIf (cfg.extraConfig != "") (lib.mkBefore cfg.extraConfig))
           (lib.concatStringsSep "\n\n" (map accountStr msmtpAccounts))
           (lib.mkIf (cfg.extraAccounts != "") (lib.mkAfter cfg.extraAccounts))