Show "Update Pending..." immediately when index button is clicked

[Copilot]

When the Album Manager loads, per-album metadata is fetched asynchronously. Clicking "Update Index" during this window produced no visible response until the async pre-flight checks completed.

Changes

• album-manager.js — startIndexing(): Immediately set button text to "Update Pending..." and disable it on click, before any async operations begin.
• Error paths: Restore original button text and re-enable on two failure cases — updateIndex() returning nothing, and corrupted-index removal failing — to leave the UI in a usable state.

The normal success flow is unaffected: showProgressUIWithoutScroll() hides the button and replaces it with the progress bar + Cancel button; on completion, handleIndexingCompletion() restores and updateAlbumCardIndexStatus() re-labels it correctly.

// Provide immediate feedback while the indexing request is being processed
const createBtn = cardElement.querySelector(".create-index-btn");
const originalBtnText = createBtn.textContent;
createBtn.textContent = "Update Pending...";
createBtn.disabled = true;

[lstein]

Works as advertised.