Patron exports: disable buttons, show “Downloading…”, prevent duplicates, restore on completion

[isaactony]

Duration. Fixes #11341

Closes #11341

Achieves: [fix]
Prevents multiple concurrent patron export requests and adds clear in-progress feedback on /account/import.

Adds openlibrary/plugins/openlibrary/js/patron_exports.js

On export form submit, disables the clicked button, sets aria-disabled, swaps CSS classes to cta-btn--unavailable cta-btn--unavailable--load, and changes text to “Downloading…”.

Uses fetch to request /account/export?type=..., reads the CSV as a Blob, and triggers download via an object URL.

Detects completion and restores the button to its original state.

Enforces a minimum 800ms visible loading period to make the state change perceptible for very fast downloads.

Wires the module in openlibrary/plugins/openlibrary/js/index.js to load only on the /account/import page.

No backend or API changes were required.

Testing

Build front-end assets:
docker compose run --rm home npm install --no-audit
docker compose run --rm home npm run build-assets:js

Run the site:
docker compose up -d
Visit http://localhost:8080/

Reproduce the fix:
Log in and navigate to /account/import
Click any “Download (.csv format)” button

Observe:
Button text changes to “Downloading…”
Button becomes disabled (greyed out)
Only one request is triggered

After download completes, button reverts to normal

@jimchamp

[jimchamp]

Thanks @isaactony, and sorry for the massive delay.

This is working as expected, but managing uploads via JS is much more complicated than I initially expected. It's highly likely that we'll roll back the JS upload management, and simply disable these buttons when they are pressed.

[jimchamp]

We typically query for elements that need hydration, then import and initialize if such elements are found on the page.

This will break if we ever move the export options to a new page.

[jimchamp]

These strings are not localized, and will appear as English despite the patron's language settings.