refactor: use username variable consistently

Replaced hardcoded "h" username references with variables:
- hosts/vm: use username variable for secrets.username
- home configs: use username variable in bash initExtra paths
- keepassxc: update comment to use $HOME instead of /home/h

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

home/modules/keepassxc.nix

@@ -7,5 +7,5 @@
       Browser.Enabled = true;
     };
   };
-  # programs.firefox.nativeMessagingHosts = [ pkgs.keepassxc ]; # FIXME: Resolve 'Access error for config file /home/h/.config/keepassxc/keepassxc.ini' error
+  # programs.firefox.nativeMessagingHosts = [ pkgs.keepassxc ]; # FIXME: Resolve 'Access error for config file $HOME/.config/keepassxc/keepassxc.ini' error
 }