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/changelog.json

@@ -1,4 +1,5 @@
 [
+  { "date": "2026-08-18", "text": "The sleep button that would just repeat the last one is now disabled: while asleep you can only tap ⏰ Sleep end, and while awake only 😴 Sleep start — no more accidental double taps creating zero-length sleep windows. If you did miss a boundary, you can still add it at the right time from the event log" },
   { "date": "2026-08-02", "text": "Added free-text notes: tap 📝 Note to jot down things that happened on a day — vaccinations, vet visits, milestones — with a date, optional photo, and any text. All your notes are collected in a new Notes section that stays visible whatever day you're viewing, so you can see at a glance when things like a tick vaccination were done" },
   { "date": "2026-08-02", "text": "The Daily counts chart now has Pees / Poos / Meals checkboxes so you can focus on just the metrics you care about — untick the rest to see, say, only poos; your choice is remembered" },
   { "date": "2026-08-01", "text": "Logging a pee or poo now sets off 💧/💩 fireworks that shoot up from the bottom of the screen — a little celebration you can switch off in Settings (and it honours a reduced-motion preference)" },