feat(dank): enable DMS/niri session on mibook, pin HM to release-26.05, drop redundant keybinds

- Pin home-manager input to release-26.05 to match nixpkgs (was resolving
  to HM master / 26.11, causing a Home Manager version mismatch warning).
- Drop niri.enableKeybinds = true from the dank HM config; keybinds are
  already provided by the imported DMS niri homeModule (includes), and using
  both together triggered the "enableKeybinds and includes.enable" warning.
- Enable my.profiles.dank on mibook (machines/mibook/environments.nix).
- Rebuild confirmed exit 0 with both target warnings resolved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018Sv2QAunLcLo3tS1YsfLgN

machines/mibook/environments.nix

@@ -6,6 +6,7 @@ in
 {
   my.profiles = {
     kde-desktop.enable = true;
+    dank.enable = true;
     zsh.enable = true;
     apps = {
       desktop_apps = true;