Jump the Review list to an artist by letter

An A–Z/# bar above the Songs and Albums lists jumps to the first artist
starting with that letter. Songs come from a new /letters endpoint giving
each leading character's offset; the song list now sorts case-insensitively
like the album list so each letter stays together.

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

internal/store/store_test.go

@@ -1,6 +1,11 @@
 package store
 
-import "testing"
+import (
+	"context"
+	"reflect"
+	"strings"
+	"testing"
+)
 
 func TestDecodeWarnings(t *testing.T) {
 	old := decodeWarnings(`["Only 3 of 9 folders match."]`)
@@ -15,3 +20,40 @@ func TestDecodeWarnings(t *testing.T) {
 		t.Fatalf("empty list should stay an empty (non-nil) list, got %#v", ws)
 	}
 }
+
+func TestMatchLetters(t *testing.T) {
+	ctx := context.Background()
+	st, err := Open(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { st.DB.Close() })
+	p := Pair{Name: "p", MasterRoot: "/m", SlaveRoot: "/s", RetentionDays: 30}
+	if err := st.SavePair(ctx, &p); err != nil {
+		t.Fatal(err)
+	}
+	for _, rel := range []string{"beck/a.flac", "ABBA/a.flac", "Air/a.flac", "2Pac/a.flac", "Björk/a.flac", "abba/b.flac"} {
+		m := Match{PairID: p.ID, RelSlave: rel, Status: StatusUnmatched, Kind: KindNone}
+		if err := st.UpsertMatch(ctx, &m); err != nil {
+			t.Fatal(err)
+		}
+	}
+	ls, err := st.MatchLetters(ctx, p.ID, MatchFilter{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []LetterStart{{"2", 0}, {"a", 1}, {"b", 4}}
+	if !reflect.DeepEqual(ls, want) {
+		t.Fatalf("letters = %+v, want %+v", ls, want)
+	}
+	// The offsets must agree with the listing order, whatever the case.
+	ms, _, err := st.Matches(ctx, p.ID, MatchFilter{Limit: 10})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, l := range ls {
+		if got := strings.ToLower(ms[l.Offset].RelSlave[:1]); got != l.Letter {
+			t.Fatalf("offset %d starts with %q, want %q (listing %v)", l.Offset, got, l.Letter, ms)
+		}
+	}
+}