Fix DownloadCollection.user.js for new Bandcamp collection page (v1.5)

[rmilev]

Summary

Bandcamp changed the collection page some time in 2025/2026 and the existing DownloadCollection.user.js no longer works: scanning finds 0–20 items, Auto-scan never completes, and the "Retry pagination" button throws.

This PR rewires the script around Bandcamp's current page model:

• The collection grid now ships only the first ~20 items in the DOM behind a view all N items button. The rest live behind POST /api/fancollection/1/collection_items (request body: {fan_id, older_than_token, count}; response: {items, redownload_urls, last_token, more_available, ...}).
• window.CollectionGrids.collection.paginate() no longer exists, so the old "Retry pagination" button is dead code.
• The album/track/download metadata is embedded server-side in document.querySelector('#pagedata').dataset.blob (item_cache.collection, collection_data.{redownload_urls,last_token,item_count}, fan_data.fan_id).

What changed

• Seed from #pagedata on script load (fan id, initial item cache, initial redownload URLs, initial pagination cursor, total item count).
• fetchAllItems() pages through the fancollection API in 20-item batches with a ~150ms gap between requests — without the gap Bandcamp returns more_available: false on the second call. Verified end-to-end on a 416-item collection.
• itemKey(item) rebuilds the {type_char}{id} key for items returned by the API (the response uses an array, not the keyed-object format that #pagedata.item_cache uses).
• findDownloadUrl(item) looks up redownload URLs by {sale_item_type}{sale_item_id} rather than always assuming a p prefix — Bandcamp keys 'p' for packages, 'a' for albums, 't' for tracks.
• Scan albums now reads from the cached items instead of scraping .collection-item-container (which only ever contains the first batch).
• Auto-scan albums calls fetchAllItems and then triggers Scan, instead of the old click-then-scroll loop that doesn't terminate on the new page.
• Retry pagination button removed — its paginate() call would throw.
• Layout: download-all / download-selected and the range row are split into separate .controls rows so they group logically.
• CSS: force readable button/input colors — Bandcamp's site styles now leave them effectively invisible.
• @version bumped to 1.5.

Test plan

• Fresh load on bandcamp.com/<your-username> with a 416-item collection
• Auto-scan ticks 40 / 416 → 60 / 416 → ... → 416 / 416
• Scan reports all downloadable items (matches non-subscription count)
• Generated download URLs match Bandcamp's own download?from=&payment_id=&sig=&sitem_id= format
• Download all / Download selected / Download range still open the right tabs