web: de-brand PhotoPrism references in user-facing copy

Replaces "PhotoPrism" in UI strings (empty states, tooltips, toasts,
log header, login screen) with neutral terms like "the indexer", "the
library", "the server" — accurate regardless of backend. The login
header becomes "Mulimage" and drops the explicit PhotoPrism mention.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/timeline/BulkActionBar.svelte

@@ -174,7 +174,7 @@
 				// a no-op.
 				if (added.length === 0) {
 					toast.error(`Nothing added to ${heap.Title}`, {
-						description: `PhotoPrism rejected all ${ids.length} UIDs (already in heap, or not indexed).`
+						description: `The server rejected all ${ids.length} UIDs (already in heap, or not indexed).`
 					});
 					return;
 				}