Disable the sleep boundary that repeats the last one

While asleep, tapping "Sleep start" again can only produce a zero-length
sleep window, and likewise "Sleep end" while awake — so renderActionHints
now disables that button outright instead of merely dimming it, with a
title explaining why ("Already asleep" / "Already awake"). Every other
action stays clickable, and a genuinely missed boundary is still fixable
from the event log, which accepts any time. With no sleep history yet,
either boundary remains a valid first event.

Also adds nodejs to the dev shell for `node --check` on src/*.js.

flake.nix

@@ -59,7 +59,9 @@
         };
 
         devShells.default = pkgs.mkShell {
-          packages = [ pkgs.go pkgs.python3 ];
+          # nodejs is here for `node --check` on src/*.js — the frontend has no
+          # build step, so this is a syntax-check tool, not a dependency.
+          packages = [ pkgs.go pkgs.python3 pkgs.nodejs ];
           shellHook = ''
             echo "puppy-tracker dev shell"
             echo "  cd server && go run . -static ../src -data /tmp/puppy-events.json"