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/style.css

@@ -272,6 +272,17 @@ button.action.note  { background: var(--note); }
    but fully tappable, so corrections are never blocked. */
 button.action.unlikely { opacity: 0.4; }
 
+/* The boundary that would repeat the latest sleep event is disabled outright
+   (see renderActionHints) — greyed out rather than merely dimmed, so it reads
+   as unavailable instead of just discouraged. */
+button.action:disabled {
+  opacity: 0.35;
+  filter: grayscale(1);
+  cursor: not-allowed;
+}
+button.action:disabled:active { transform: none; }
+button.action:disabled:hover { filter: grayscale(1); }
+
 button.ghost {
   background: transparent;
   color: var(--text);