Drop Artist/Album/Title depth assumptions from layout checks

Matching has always been by relative path, so any folder layout works as
long as both libraries use the same one. Remove the depth and loose-root-file
warnings, which flagged valid layouts, and reword the UI and README to say
"same folder layout on both sides" with Artist/Album/Title.ext only as an
example. The "elsewhere" fallback is now described as same top-level folder,
different subfolder. Add a test covering flat and deeply nested layouts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/pages/Build.tsx

@@ -39,8 +39,8 @@ export function Build({ pair }: { pair: Pair }) {
   return (
     <div className="space-y-4">
       <Alert tone="blue">
-        Build the slave library from the host. Browse the host library, pick artists, albums or songs, and playlink hardlinks them into the slave
-        library at the same <code className="mono">Artist/Album/Title.ext</code> path. No extra space is used. Existing slave files are never
+        Build the slave library from the host. Browse the host library, pick folders or songs, and playlink hardlinks them into the slave
+        library at the same relative path. No extra space is used. Existing slave files are never
         overwritten, and <b>Remove</b> only deletes slave files that are hardlinks of the host file, so the host copy is always kept.
       </Alert>
 

web/src/pages/PairForm.tsx

@@ -91,7 +91,8 @@ export function PairForm({ pair, open, onClose }: { pair?: Pair; open: boolean;
           />
         </div>
         <Alert tone="blue">
-          Both libraries should use the same <code className="mono">Artist/Album/Title.ext</code> layout. Files are matched by relative path first.
+          Both libraries should use the same folder layout (for example <code className="mono">Artist/Album/Title.ext</code>), because files are matched by
+          their relative path first. Any layout works as long as it is the same on both sides.
           Both must be on the same filesystem. On Unraid, mount one common parent folder into the container instead of two separate volumes.
         </Alert>
         <PathField

web/src/pages/Review.tsx

@@ -20,7 +20,7 @@ const kindLabel: Record<Match["kind"], string> = {
   path_case: "path differs in case",
   extension: "other format",
   fuzzy: "similar title",
-  elsewhere: "other album folder",
+  elsewhere: "same top folder, other subfolder",
   none: "no match",
 };