fix(playlists): thread the source provider id through to recent playlists

Fixes #368.

Reopening a Spotify playlist from "recent access" showed no tracks,
while the first view (right after pasting the URL) worked fine.

Root cause traced across two layers:

1. Go: ExtURLHandleResult (the parsed shape of an extension's handleUrl()
   return value) never captured a top-level `id` for the handled
   resource. Track/album/artist results carry their own id inside their
   nested metadata, but a plain playlist result has no such object, so
   its id was silently dropped everywhere from the goja parser through
   to the JSON the Dart side receives.

2. Dart: TrackState had nowhere to put that id even if it existed
   (only playlistName), so recording a "recent" playlist access stored
   the playlist's *name* as if it were its id. Reopening it later fed
   that name into PlaylistScreen's provider-guessing logic
   (legacyProviderIdFromResourceId, which only recognizes legacy
   "provider:id" prefixes), which naturally failed and fell through to
   a hardcoded Deezer metadata fetch using a Spotify playlist's name as
   the resource id - guaranteed to return nothing.

Fix, matching the "generic API, not per-provider checks" architecture
in CONTRIBUTING.md:

- go_backend/extension_provider_wrapper.go: add ExtURLHandleResult.ID.
- go_backend/extension_goja_convert.go: parse it from the handler's
  return value.
- go_backend/exports_extensions.go: surface it in the JSON response.
- lib/providers/track_provider.dart: add TrackState.playlistId,
  populated from the response's `id` field for playlist results.
- lib/screens/home_tab.dart: record the real playlist id (falling back
  to the name only if a provider never supplies one) and pass both the
  id and the already-known provider id forward to PlaylistScreen.
- lib/screens/playlist_screen.dart: PlaylistScreen gains a
  metadataProviderId param that takes priority over guessing from the
  id's shape.
- lib/screens/home_tab_recent.dart: pass the recent-access entry's
  stored providerId through when reopening a playlist.
- lib/utils/provider_resource_ids.dart: extract the
  known-id-vs-guessed-id preference into a small, directly testable
  resolvePreferredMetadataProviderId helper.

Note: this fixes the common case (an extension already reported its
own id as the source provider for the URL it handled). If a provider
never supplies an id for its handleUrl() playlist result, playlistId
stays null and behavior is unchanged from before this fix - no
regression, just not a complete fix for that narrower case, since
that would require changes in extension-side JS code outside this
repo.

Tests:
- go_backend/extension_goja_convert_url_handle_test.go: the new ID
  field round-trips from a handler's return value, and stays empty
  when the handler doesn't supply one.
- test/provider_resource_ids_test.dart: resolvePreferredMetadataProviderId
  prefers a known provider id, falls back to the legacy-prefix guess,
  treats a blank known id as unknown, and returns null for the
  unprefixed-id-with-no-known-provider case from #368 itself.

Verification: go build/vet/test and flutter analyze/test all green.

go_backend/exports_extensions.go

@@ -805,6 +805,7 @@ func HandleURLWithExtensionJSON(url string) (string, error) {
 
 	response := map[string]any{
 		"type":         result.Type,
+		"id":           result.ID,
 		"extension_id": extensionID,
 		"name":         result.Name,
 		"cover_url":    result.CoverURL,

go_backend/extension_goja_convert.go

@@ -522,6 +522,7 @@ func parseExtensionURLHandleValue(vm *goja.Runtime, value goja.Value) (ExtURLHan
 	obj := value.ToObject(vm)
 	handleResult := ExtURLHandleResult{
 		Type:        gojaObjectString(obj, "type"),
+		ID:          gojaObjectString(obj, "id"),
 		Name:        gojaObjectString(obj, "name"),
 		CoverURL:    gojaObjectString(obj, "cover_url", "coverUrl"),
 		HeaderImage: gojaObjectString(obj, "header_image", "headerImage"),

go_backend/extension_goja_convert_url_handle_test.go

@@ -0,0 +1,53 @@
+package gobackend
+
+import (
+	"testing"
+
+	"github.com/dop251/goja"
+)
+
+func TestParseExtensionURLHandleValueCapturesID(t *testing.T) {
+	vm := goja.New()
+	value, err := vm.RunString(`({
+		type: "playlist",
+		id: "37i9dQZF1DXcBWIGoYBM5M",
+		name: "Discover Weekly",
+		tracks: []
+	})`)
+	if err != nil {
+		t.Fatalf("RunString: %v", err)
+	}
+
+	result, err := parseExtensionURLHandleValue(vm, value)
+	if err != nil {
+		t.Fatalf("parseExtensionURLHandleValue: %v", err)
+	}
+	if result.Type != "playlist" {
+		t.Fatalf("Type = %q", result.Type)
+	}
+	if result.ID != "37i9dQZF1DXcBWIGoYBM5M" {
+		t.Fatalf("ID = %q, want the playlist id from the handler result", result.ID)
+	}
+	if result.Name != "Discover Weekly" {
+		t.Fatalf("Name = %q", result.Name)
+	}
+}
+
+func TestParseExtensionURLHandleValueOmitsIDWhenAbsent(t *testing.T) {
+	vm := goja.New()
+	// Track/album/artist results carry their own ID inside their nested
+	// metadata, so a handler that omits the top-level "id" (as these do
+	// today) must not surface a stale or zero-value ID.
+	value, err := vm.RunString(`({ type: "track", name: "A Track" })`)
+	if err != nil {
+		t.Fatalf("RunString: %v", err)
+	}
+
+	result, err := parseExtensionURLHandleValue(vm, value)
+	if err != nil {
+		t.Fatalf("parseExtensionURLHandleValue: %v", err)
+	}
+	if result.ID != "" {
+		t.Fatalf("ID = %q, want empty when the handler result has none", result.ID)
+	}
+}

go_backend/extension_provider_wrapper.go

@@ -724,7 +724,11 @@ func (p *extensionProviderWrapper) customSearch(query string, options map[string
 }
 
 type ExtURLHandleResult struct {
-	Type        string             `json:"type"`
+	Type string `json:"type"`
+	// ID identifies the handled resource itself (e.g. a playlist ID). Track,
+	// album, and artist results already carry their own ID inside their
+	// nested metadata; this covers result types with no such nested object.
+	ID          string             `json:"id,omitempty"`
 	Track       *ExtTrackMetadata  `json:"track,omitempty"`
 	Tracks      []ExtTrackMetadata `json:"tracks,omitempty"`
 	Album       *ExtAlbumMetadata  `json:"album,omitempty"`