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

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

hosts/vm/default.nix

@@ -33,7 +33,7 @@ in
     })
   ];
 
-  secrets.username = "h";
+  secrets.username = username;
 
   environment.systemPackages = [ inputs.nvim.packages.x86_64-linux.nvim ];