fix(dlna): always return non-nil device slice so API serializes [] not null

The DLNA device list endpoint could return {"devices":null} in two cases:
the SSDP-failure path returned a nil slice, and the cached path copied an
empty cache via append([]DLNADevice(nil), cache...) which also yields nil.
The web UI reads res.devices.length, so a null devices field crashed the
DLNA page to a white screen. Return non-nil slices in both paths and guard
the frontend with a null coalesce. Adds a regression test.

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal/service/dlna.go

@@ -114,7 +114,8 @@ func (d *DLNAService) ssdpDiscover(ctx context.Context, timeout time.Duration) (
 func (d *DLNAService) Discover(ctx context.Context, force bool) ([]DLNADevice, error) {
 	d.mu.Lock()
 	if !force && time.Since(d.cachedAt) < 30*time.Second && d.cache != nil {
-		out := append([]DLNADevice(nil), d.cache...)
+		// Copy into a non-nil slice so an empty cache serializes as [] not null.
+		out := append(make([]DLNADevice, 0, len(d.cache)), d.cache...)
 		d.mu.Unlock()
 		return out, nil
 	}
@@ -123,9 +124,10 @@ func (d *DLNAService) Discover(ctx context.Context, force bool) ([]DLNADevice, e
 	locations, err := d.ssdpDiscover(ctx, 3*time.Second)
 	if err != nil {
 		// SSDP often fails on container networks; treat as "no devices"
-		// rather than 500 the API.
+		// rather than 500 the API. Return an empty (non-nil) slice so the
+		// JSON response is [] not null.
 		d.log.Debug("ssdp discover failed", zap.Error(err))
-		return nil, nil
+		return []DLNADevice{}, nil
 	}
 	devices := make([]DLNADevice, 0, len(locations))
 	for _, loc := range locations {

internal/service/dlna_test.go

@@ -0,0 +1,37 @@
+package service
+
+import (
+	"context"
+	"encoding/json"
+	"testing"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+// TestDLNADiscoverCachedEmptyReturnsNonNil guards against the bug where an
+// empty device cache was copied via append([]DLNADevice(nil), cache...),
+// which yields a nil slice and serializes to JSON null. The web UI reads
+// res.devices.length, so a null devices field crashed the DLNA page.
+func TestDLNADiscoverCachedEmptyReturnsNonNil(t *testing.T) {
+	d := NewDLNAService(zap.NewNop())
+	// Prime the cache with an empty (but non-nil) result, fresh enough to be served.
+	d.cache = []DLNADevice{}
+	d.cachedAt = time.Now()
+
+	out, err := d.Discover(context.Background(), false)
+	if err != nil {
+		t.Fatalf("Discover returned error: %v", err)
+	}
+	if out == nil {
+		t.Fatal("Discover returned nil slice for empty cache; want non-nil so JSON is [] not null")
+	}
+
+	b, err := json.Marshal(map[string]any{"devices": out})
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	if got := string(b); got != `{"devices":[]}` {
+		t.Fatalf("devices serialized as %s; want {\"devices\":[]}", got)
+	}
+}

web/src/api/dlna.ts

@@ -13,8 +13,8 @@ export interface DLNADevice {
 export const dlnaAPI = {
   list: (force = false) =>
     api
-      .get<{ devices: DLNADevice[] }>('/dlna/devices', { params: { force: force ? 'true' : '' } })
-      .then((r) => r.data.devices),
+      .get<{ devices: DLNADevice[] | null }>('/dlna/devices', { params: { force: force ? 'true' : '' } })
+      .then((r) => r.data.devices ?? []),
 
   cast: (controlURL: string, mediaURL: string) =>
     api