fix: rename duplicate clearBtn variable to clearFocusBtn in renderFoc…

[akhilmodi29]

Fixes #693

Problem
The clearBtn variable was declared twice in app.js:

• First at the top level:
  const clearBtn = document.getElementById('clear-btn')
• Second inside renderFocusTasks():
  const clearBtn = activeFocusTask.querySelector('.clear-focus-task-btn')

This variable shadowing caused:

• The inner clearBtn shadowing the outer clearBtn
• Clear button event listeners potentially conflicting
• Confusing and error-prone code

Fix
Renamed the inner clearBtn to clearFocusBtn inside
renderFocusTasks() to eliminate the variable shadowing.
Updated all 3 references in that block accordingly.

Testing

• Clear button in focus task works correctly ✅
• Main clear button still works correctly ✅
• No variable shadowing in app.js ✅

[akhilmodi29]

@Charushi06 Please review this PR and add the required labels including gssoc:approved. This fixes the duplicate clearBtn variable shadowing bug in app.js.

Labels suggested:

• level:beginner
• type:bug
• quality:clean