web: send the workout "left" signal when the page goes away, and say why

[kurktchiev]

The Workout screen sent its "left" signal (POST /api/activity { active: false }, which drops the athlete from the admin dashboard's "Training now") only
from its effect cleanup. That runs on in-app navigation, where the api()
call beside it sends the same thing anyway. A closed tab, or a home-screen
app swiped away or killed by the phone, unmounts nothing, so the athlete
stayed under "Training now" until the server's presence expiry, 70 s after
the last heartbeat.

The screen now sends it through beacon() (still navigator.sendBeacon,
unchanged) on pagehide, and on visibilitychange to hidden as well,
because iOS kills a hidden home-screen app without a pagehide. The signal
now carries a reason the server acts on: navigated from the effect
cleanup, closed from pagehide, hidden from visibilitychange. The
server keeps a hidden athlete listed for 45 s unless a heartbeat arrives.
A phone locked between sets keeps heartbeating and is never blinked off the
list, while an iOS app swiped away is gone in 45 s instead of the full 70 s
expiry. Each reason is sent once per hide, re-armed by a heartbeat; a close
whose pagehide follows a hidden visibilitychange still sends closed,
and nothing follows closed. A page shown again, or restored from the
back/forward cache, heartbeats at once.

api 7 new tests (server-presence.test.js; a new test/fake-clock.mjs
drives the spawned server's clock over IPC so each step in the test lands at
an exact server time), run with node --test. frontend 10 new tests in
Workout.test.jsx, run with npx vitest run --maxWorkers=2. 129 total in
that file, all passing.

Tested in real Chromium 153 and WebKit 26.6 through Playwright. With a bare page and a
server that logs what arrives, the beacon arrived on every tab close in both browsers (18 of
18, and 10 of 10 with 150 ms of added latency) and on every navigation away. In a separate
run of the full app with WebKit on macOS, a tab close lost the request in 4 of 4 tries, and
a keepalive fetch in place of the beacon lost it the same way, so that may be Playwright
tearing the page down rather than Safari. The hidden reason is what covers a home-screen
app iOS suspends or kills, which Playwright cannot exercise.

Conflicts with #312 (generic prescription engine v2) in Workout.test.jsx,
where both add new test blocks at the same places. Keeping both resolves it.

🤖 Generated with Claude Code