fix: wait for stop button visibility before clicking in E2E test

[abhnish]

Description

Fixes a flaky Cypress E2E test in main.cy.js where cy.get("#stop").click()
was called immediately after clicking play, without waiting for the stop
button to become visible.

The stop button has display: none by default and only appears after the
program starts running. Adding .should("be.visible") before .click()
tells Cypress to wait until the button is visible before interacting with it,
eliminating the timing-dependent failure.

Related Issue

N/A — pre-existing test flakiness, not tied to a specific issue.

Category

• Bug Fix

Type of Change

• Bug Fix

Testing Performed

1. Change is a one-line addition of .should("be.visible") before .click().
2. No app logic touched — test file only.

PR Checklist

• My changes generate no new warnings or console errors
• I have checked for and resolved any merge conflicts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.94%. Comparing base (eb2c25b) to head (7163b7d).
⚠️ Report is 55 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7677   +/-   ##
=======================================
  Coverage   51.94%   51.94%           
=======================================
  Files         169      169           
  Lines       56826    56826           
=======================================
  Hits        29519    29519           
  Misses      27307    27307           

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[github-actions]

This PR has merge conflicts with master.

Please rebase your branch:

# Add upstream remote (one-time setup)
git remote add upstream https://github.com/sugarlabs/musicblocks.git

# Fetch latest master and rebase
git fetch upstream
git rebase upstream/master

# Resolve any conflicts, then:
git push --force-with-lease origin YOUR_BRANCH

Tip: Enable "Allow edits from maintainers" on this PR so we can auto-rebase for you next time. This only grants access to your PR branch. Your fork's other branches are not affected.