fix(useInterval): clear a manually resumed interval on unmount

[ostapondo]

Description

useInterval never clears its timer on unmount when controls: true is used.

The effect that owns the cleanup bails out before it can register one:

if (controls) {
  return
}

So an interval started through resume() keeps firing after the component is
gone. The hook's docs promise the opposite:

The interval is automatically cleared on component unmount.

To reproduce: mount with { controls: true }, call resume(), unmount, then
advance the timers. The callback keeps running — 10 more calls over 500ms at a
50ms delay in the test I added.

I put the unmount cleanup in its own mount-scoped effect rather than returning
clean from the existing one. That keeps the current delay-change behaviour
untouched: in manual mode, updating delay still doesn't stop a running timer,
which felt like a separate decision from fixing the leak.

While I was in there — resume() overwrote timer.current without clearing the
previous timer, so calling it twice orphaned the first interval and neither
pause() nor the unmount cleanup could reach it afterwards. It now clears first.

Type of Change

• Bug fix
• New hook
• Enhancement to existing hook
• Documentation update
• Other (please describe)

Checklist

• I have read the Contributing Guide
• My code follows the project's coding style
• I have added tests for my changes
• All existing tests pass
• I have updated the documentation

No doc change: the existing page already describes the fixed behaviour.

Two tests added to useInterval/index.spec.ts — one for the unmount leak, one
for the double-resume() orphan. Both fail on main and pass here; the other 9
in that file are untouched.