Show scan warnings as structured cards instead of long sentences

Warnings are now stored and served as structured data (title, detail,
slave/master items, total) and rendered as cards with a Slave -> Master
table: the first five rows, with "Show all" for the rest.

- Renamed folders highlight the characters that differ; non-ASCII
  characters get their code point in a tooltip, so look-alikes such as
  "-" (U+002D) and "‐" (U+2010) can be told apart.
- Songs in a different folder are grouped by folder pair with a song
  count, most affected first, instead of two long examples.
- Folders that only differ by name are no longer also reported as
  songs in a different folder.
- Warnings saved as plain text by earlier scans still display.

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

web/src/api.ts

@@ -75,7 +75,7 @@ export type Pair = {
   approx_threshold: number;
   last_scan_at: number | null;
   last_scan_error: string;
-  last_scan_warnings: string[];
+  last_scan_warnings: ScanWarning[];
   summary: Summary;
   job: Job | null;
 };
@@ -199,3 +199,10 @@ export type BulkPreview = {
   bytes_freed: number;
   examples: Record<string, string[] | undefined>;
 };
+
+export type ScanWarning = {
+  title: string;
+  detail?: string;
+  items?: { slave: string; master: string; count?: number }[];
+  total?: number;
+};