Fix download button disabled state

[Pragzh]

What changed

• Disabled the Download Notes button when "All Subjects" is selected.
• Added helper text to guide users to select a subject before downloading.

Why

Previously, clicking Download when "All Subjects" was selected did nothing and gave no feedback.

Testing

• Ran npm run build successfully.
• Checked that the button is disabled when "All Subjects" is selected.
• Checked that the button works after selecting a subject.

Fixes #110

[vercel]

@Pragzh is attempting to deploy a commit to the Lovely Mahour's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Pragzh]

Hi @maintainer, my PR checks are failing because Vercel deployment needs team authorization. Could you please authorize the Vercel check when possible? Thank you!