feat(server): serve embedded SPA at root with /api,/rest carve-out

- NotFound wrapper routes unknown /api/* and /rest/* to a JSON 404
- Everything else falls through to the embedded web handler, which
  resolves static assets or returns index.html for SPA deep links

internal/server/server_test.go

@@ -6,6 +6,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/subsonic"
@@ -33,3 +34,79 @@ func TestHealthz(t *testing.T) {
 		t.Errorf("body = %v, want status=ok", body)
 	}
 }
+
+func TestRouter_ServesSPAAtRoot(t *testing.T) {
+	s := New(slog.New(slog.NewTextHandler(io.Discard, nil)), nil, nil, subsonic.Config{})
+	ts := httptest.NewServer(s.Router())
+	defer ts.Close()
+
+	resp, err := http.Get(ts.URL + "/")
+	if err != nil {
+		t.Fatalf("GET /: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("status = %d, want 200", resp.StatusCode)
+	}
+	if ct := resp.Header.Get("Content-Type"); !strings.Contains(ct, "text/html") {
+		t.Errorf("Content-Type = %q, want text/html*", ct)
+	}
+}
+
+func TestRouter_DeepLinkFallbackReturnsSPA(t *testing.T) {
+	s := New(slog.New(slog.NewTextHandler(io.Discard, nil)), nil, nil, subsonic.Config{})
+	ts := httptest.NewServer(s.Router())
+	defer ts.Close()
+
+	resp, err := http.Get(ts.URL + "/artists/00000000-0000-0000-0000-000000000001")
+	if err != nil {
+		t.Fatalf("GET deep link: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Errorf("status = %d, want 200", resp.StatusCode)
+	}
+	if ct := resp.Header.Get("Content-Type"); !strings.Contains(ct, "text/html") {
+		t.Errorf("Content-Type = %q, want text/html*", ct)
+	}
+}
+
+func TestRouter_APIPathNotSwallowedBySPA(t *testing.T) {
+	s := New(slog.New(slog.NewTextHandler(io.Discard, nil)), nil, nil, subsonic.Config{})
+	ts := httptest.NewServer(s.Router())
+	defer ts.Close()
+
+	resp, err := http.Get(ts.URL + "/api/does-not-exist")
+	if err != nil {
+		t.Fatalf("GET /api/does-not-exist: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusNotFound {
+		t.Errorf("status = %d, want 404", resp.StatusCode)
+	}
+	if ct := resp.Header.Get("Content-Type"); strings.Contains(ct, "text/html") {
+		t.Errorf("Content-Type = %q; /api/* must never return text/html", ct)
+	}
+}
+
+func TestRouter_RestPathNotSwallowedBySPA(t *testing.T) {
+	s := New(slog.New(slog.NewTextHandler(io.Discard, nil)), nil, nil, subsonic.Config{})
+	ts := httptest.NewServer(s.Router())
+	defer ts.Close()
+
+	resp, err := http.Get(ts.URL + "/rest/does-not-exist")
+	if err != nil {
+		t.Fatalf("GET /rest/does-not-exist: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusNotFound {
+		t.Errorf("status = %d, want 404", resp.StatusCode)
+	}
+	if ct := resp.Header.Get("Content-Type"); strings.Contains(ct, "text/html") {
+		t.Errorf("Content-Type = %q; /rest/* must never return text/html", ct)
+	}
+}