fix(sheets): pin and un-pin the page in the same commit as the sheet, and never scroll for a sheet's own history pop

[kurktchiev]

One commit. Two small changes that have to go together, because each on its own only moves the jump
around.

What breaks

Reported from a phone, mid-session. Picking an RIR level, one tap that closes the picker sheet and
ticks the set, made the page "jump down to the bottom and back up to the exercise". A plain tick did
not, so it was the sheet.

Modals pins the body behind a sheet (position: fixed; top: -y) and puts the scroll back when the
sheet goes. Both ran in a passive effect, after paint. iOS applies scrolls asynchronously, so there
was a frame with the body un-pinned at scroll 0 before the restore, and the 350 ms keyboard retry then
pulled the page back up. That is the jump, and back.

App.jsx made it worse from the other side. Its back-navigation scroll restore fires on every POP,
and Modals pushes a history entry per sheet so the system back button closes it. Closing a sheet is
therefore a POP that lands on the screen you are already on, and the restore read the recorded
position a frame later and added a scroll of its own on top of the sheet's.

Reproduce

On iOS, scrolled some way down the workout screen, tick a set with effort ratings on. The picker
opens, you pick, and the page jumps to the bottom and comes back.

What changed

• The pin is a useLayoutEffect, so the un-pin and the restore land in the same frame as the sheet
  leaving. There is no frame at scroll 0 to see.
• The restore is said once more on the next animation frame, guarded on the body no longer being
  pinned, because iOS can apply a scroll issued in the same task a frame late or against the
  still-short layout.
• App.jsx ignores a POP that lands on the same path. That is a sheet's history entry going away, not
  a page change, and it has no scroll of its own to restore.

The 350 ms keyboard retry that was already there stays; a sheet closed with the keyboard still up (tap
"+" in the picker, then finish) makes iOS scroll the page again after everything above has run.

Tests

Modals.test.jsx, one case: the body is pinned on open with the scroll position folded into top,
and on close the scroll is restored three times over, in order. At once, in the same commit; again on
the next animation frame; and again after the keyboard's dismiss animation. Against main it fails on
the second of those, because the passive effect only ever issued one.

Desktop WebKit paints the un-pin and the restore together, so it does not show the symptom. The test
pins the order rather than the pixels.

Checklist

• npx vitest run passes in frontend/: 1585 passing across 126 files (main is 1584; +1)
• npm test in api/ untouched by this PR
• No new runtime dependency
• No user-facing strings, so no locale change
• CHANGELOG.md is left alone

🤖 Generated with Claude Code

[DuarteSantos8]

Thanks @kurktchiev, no more jump on iOS when a sheet closes.

Released in v1.3.9: https://github.com/DuarteSantos8/openGym/releases/tag/v1.3.9