feat: M3.5 polish — genre splitting + radio button surface

Two carry-overs from M3 verification, bundled with the search-input
fix already on dev (b7a59a9).

1. Genre splitting in BuildSessionVector
   The library has many tracks whose genre tag is a denormalized
   multi-genre string ("Indie Pop; Pop; Alternative Pop"). Reading them
   as one opaque tag means a single-genre "Pop" track and a multi-genre
   track listing Pop both fail to share the Pop key, so the tags axis
   in similarity scoring (weight 0.7!) returned 0 in nearly all cases
   on real libraries. Result: contextual scoring couldn't differentiate.

   Add splitGenres() that splits on ; and , and trims whitespace;
   strings without a delimiter come back as a single-element slice
   (so the existing single-genre cases keep working unchanged).
   Concatenated-without-separator output ("ElectronicComplextroGlitch
   Hop") stays opaque — that needs a genre dictionary, out of scope.

2. Play-radio button on TrackRow
   playRadio() was only wired to the click handler on /search and
   /search/tracks, leaving /library/liked, album pages, and search
   results' inner clicks with no way to start a radio. Adds a small
   radio button to TrackRow (between LikeButton and the +queue button).
   Click → playRadio(track.id), with stopPropagation so the row's own
   activate handler doesn't also fire.

Tests:
- 3 new sessionvector tests: TestSplitGenres, multi-genre semicolon
  split, no-separator stays opaque. Existing single-genre tests pass
  unchanged.
- 1 new TrackRow test: radio button calls playRadio with track id and
  does NOT trigger row play.

Verified locally: go -short -race ./... clean, golangci-lint clean,
svelte-check 0/0, 175 vitest tests (was 174), web build succeeds.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/recommendation/sessionvector.go

@@ -1,6 +1,8 @@
 package recommendation
 
 import (
+	"strings"
+
 	"github.com/jackc/pgx/v5/pgtype"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
@@ -28,7 +30,9 @@ func BuildSessionVector(priorTracks []dbq.Track) SessionVector {
 			v.Artists = append(v.Artists, artistID)
 		}
 		if t.Genre != nil && *t.Genre != "" {
-			v.Tags[*t.Genre]++
+			for _, g := range splitGenres(*t.Genre) {
+				v.Tags[g]++
+			}
 		}
 		v.RecentTrackIDs = append(v.RecentTrackIDs, uuidString(t.ID))
 	}
@@ -41,3 +45,22 @@ func uuidString(u pgtype.UUID) string {
 	}
 	return u.String()
 }
+
+// splitGenres splits a track's denormalized genre string on the common
+// multi-genre delimiters (`;`, `,`) used by various tag editors. Trims
+// whitespace; drops empty fragments. Strings with no delimiter come back
+// as a single-element slice. Concatenated-without-separator inputs (e.g.
+// "ElectronicComplextroGlitch Hop" from broken tag-editor output) cannot
+// be split without a genre dictionary and stay as one opaque tag.
+func splitGenres(s string) []string {
+	parts := strings.FieldsFunc(s, func(r rune) bool {
+		return r == ';' || r == ','
+	})
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		if p = strings.TrimSpace(p); p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}

internal/recommendation/sessionvector_test.go

@@ -123,6 +123,77 @@ func TestBuildSessionVector_NilGenre_NotIndexed(t *testing.T) {
 	}
 }
 
+func TestBuildSessionVector_MultiGenreSplitsOnSemicolon(t *testing.T) {
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	v := BuildSessionVector([]dbq.Track{
+		track(a1, "Indie Pop; Pop; Alternative Pop"),
+		track(a1, "Pop"),
+		track(a1, "Boom Bap; Hip Hop; Lo-Fi"),
+	})
+	// Pop appears once on track 1 and once on track 2 → count 2.
+	if v.Tags["Pop"] != 2 {
+		t.Errorf("Tags['Pop'] = %d, want 2 (one per matching track)", v.Tags["Pop"])
+	}
+	// Indie Pop, Alternative Pop only on track 1.
+	if v.Tags["Indie Pop"] != 1 || v.Tags["Alternative Pop"] != 1 {
+		t.Errorf("Indie/Alt Pop counts wrong: %+v", v.Tags)
+	}
+	// Track 3's genres.
+	if v.Tags["Boom Bap"] != 1 || v.Tags["Hip Hop"] != 1 || v.Tags["Lo-Fi"] != 1 {
+		t.Errorf("track-3 genre counts wrong: %+v", v.Tags)
+	}
+}
+
+func TestBuildSessionVector_NoSeparatorStaysOpaque(t *testing.T) {
+	// Concatenated strings without separators (broken tag-editor output)
+	// can't be parsed; they stay as one tag and only match identical strings.
+	a1 := uuid("11111111-1111-1111-1111-111111111111")
+	v := BuildSessionVector([]dbq.Track{
+		track(a1, "ElectronicComplextroGlitch Hop"),
+		track(a1, "ElectronicComplextroGlitch Hop"),
+	})
+	if v.Tags["ElectronicComplextroGlitch Hop"] != 2 {
+		t.Errorf("opaque tag count = %d, want 2", v.Tags["ElectronicComplextroGlitch Hop"])
+	}
+	if len(v.Tags) != 1 {
+		t.Errorf("len(Tags) = %d, want 1", len(v.Tags))
+	}
+}
+
+func TestSplitGenres(t *testing.T) {
+	cases := []struct {
+		in   string
+		want []string
+	}{
+		{"", nil},
+		{"Pop", []string{"Pop"}},
+		{"Pop; Rock", []string{"Pop", "Rock"}},
+		{"Pop, Rock", []string{"Pop", "Rock"}},
+		{"Pop; Rock, Jazz", []string{"Pop", "Rock", "Jazz"}},
+		{"  Pop  ;  Rock  ", []string{"Pop", "Rock"}},
+		{";; Pop ;;", []string{"Pop"}},
+		{"ElectronicComplextroGlitch Hop", []string{"ElectronicComplextroGlitch Hop"}},
+	}
+	for _, c := range cases {
+		got := splitGenres(c.in)
+		if c.want == nil {
+			if len(got) != 0 {
+				t.Errorf("splitGenres(%q) = %v, want empty", c.in, got)
+			}
+			continue
+		}
+		if len(got) != len(c.want) {
+			t.Errorf("splitGenres(%q) = %v, want %v", c.in, got, c.want)
+			continue
+		}
+		for i := range got {
+			if got[i] != c.want[i] {
+				t.Errorf("splitGenres(%q)[%d] = %q, want %q", c.in, i, got[i], c.want[i])
+			}
+		}
+	}
+}
+
 func TestBuildSessionVector_JSONRoundTrip(t *testing.T) {
 	a1 := uuid("11111111-1111-1111-1111-111111111111")
 	v := BuildSessionVector([]dbq.Track{