feat: Wikidata engine and Wikipedia knowledge infobox

- Add wikidata engine (wbsearchentities), tests, factory/planner/config
- Wikipedia REST summary: infobox from extract, thumbnail, article URL
- InfoboxView URL; render infobox list in results_inner + base styles
- Preferences Wikidata toggle; engine badge color for wikidata

Made-with: Cursor

internal/engines/wikidata_test.go

@@ -0,0 +1,51 @@
+package engines
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/metamorphosis-dev/samsa/internal/contracts"
+)
+
+func TestWikidataEngine_Search(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Query().Get("action") != "wbsearchentities" {
+			t.Errorf("action=%q", r.URL.Query().Get("action"))
+		}
+		if got := r.URL.Query().Get("search"); got != "test" {
+			t.Errorf("search=%q want test", got)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"search":[{"id":"Q937","label":"Go","description":"Programming language"}]}`))
+	}))
+	defer ts.Close()
+
+	orig := wikidataAPIBase
+	t.Cleanup(func() { wikidataAPIBase = orig })
+	wikidataAPIBase = ts.URL + "/w/api.php"
+
+	e := &WikidataEngine{client: ts.Client()}
+	resp, err := e.Search(context.Background(), contracts.SearchRequest{
+		Query:    "test",
+		Language: "en",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(resp.Results) != 1 {
+		t.Fatalf("expected 1 result, got %d", len(resp.Results))
+	}
+	r0 := resp.Results[0]
+	if r0.Engine != "wikidata" {
+		t.Errorf("engine=%q", r0.Engine)
+	}
+	if r0.Title != "Go" {
+		t.Errorf("title=%q", r0.Title)
+	}
+	if r0.URL == nil || !strings.Contains(*r0.URL, "Q937") {
+		t.Errorf("url=%v", r0.URL)
+	}
+}