Daily step reset happens at UTC midnight instead of the user's local midnight #10

Problem

The daily auto-reset of steps/distance/calories fires at UTC midnight, not the user's local midnight. Any user not on UTC has their counters reset in the middle of the day.

Root cause

app.js:

function today() { return new Date().toISOString().slice(0, 10); }   // toISOString() is UTC

load() then does the daily reset based on this UTC date:

if (state.lastReset !== today()) {
  state.steps = 0; state.gpsDistM = 0; state.gpsSegments = 0;
  state.lastReset = today();
  state.running = false;
  save();
}

Because toISOString() returns the UTC calendar date, the "new day" boundary is 00:00 UTC. For example, a US-Pacific user's steps reset around 16:00–17:00 their local time.

Expected

Counters reset at the user's local midnight.

Suggested fix

Compute the local date instead of the UTC date, e.g.:

function today() {
  var d = new Date();
  return d.getFullYear() + '-' +
         String(d.getMonth() + 1).padStart(2, '0') + '-' +
         String(d.getDate()).padStart(2, '0');
}

(or new Date().toLocaleDateString('en-CA'), which yields YYYY-MM-DD in local time). lastReset is stored/compared with the same function, so only today() needs changing.