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/search/request_params.go

@@ -31,7 +31,7 @@ const maxQueryLength = 1024
 
 // knownEngineNames is the allowlist of valid engine identifiers.
 var knownEngineNames = map[string]bool{
-	"wikipedia": true, "arxiv": true, "crossref": true,
+	"wikipedia": true, "wikidata": true, "arxiv": true, "crossref": true,
 	"braveapi": true, "brave": true, "qwant": true,
 	"duckduckgo": true, "github": true, "reddit": true,
 	"bing": true, "google": true, "youtube": true,