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.

src/app.js

@@ -1989,17 +1989,30 @@
   }
 
   // Dim the quick actions that don't fit the current sleep state — a nudge
-  // toward the likely next tap, never a block: everything stays clickable so
-  // corrections (a missed sleep-end, a mid-nap pee) are always possible.
+  // toward the likely next tap. The one boundary that would just repeat the
+  // latest sleep event (a second "sleep start" while already asleep, or a
+  // second "sleep end" while already awake) is disabled outright, since it
+  // can only produce a zero-length window. Everything else stays clickable so
+  // corrections (a mid-nap pee) are never blocked, and a genuinely missed
+  // boundary is still fixable from the event log, which accepts any time.
   function renderActionHints(events) {
     const { state } = currentSleepState(events);
     document.querySelectorAll("button.action").forEach(btn => {
       const type = btn.dataset.type;
-      const unlikely =
+      const repeat =
+        state === "asleep" ? type === "sleep-start"
+        : state === "awake" ? type === "sleep-end"
+        : false; // no sleep history yet — either boundary is a fine first event
+      const unlikely = !repeat && (
         state === "asleep" ? type !== "sleep-end"
         : state === "awake" ? type === "sleep-end"
-        : false; // no sleep history yet — no hints to give
+        : false // no sleep history yet — no hints to give
+      );
       btn.classList.toggle("unlikely", unlikely);
+      btn.disabled = repeat;
+      btn.title = repeat
+        ? (type === "sleep-start" ? "Already asleep" : "Already awake")
+        : "";
     });
   }