Keep launcher locale changes from mutating shared web-search routing (#2573)

The launcher wired UI language changes into a process-global backend
switch that changed auto web-search provider selection and the
reported current service for every handler in the same process.

This narrows the fix to the validated leak: remove backend sync from
frontend locale changes, drop the now-unused UI endpoint, and make
auto selection fall back to a stable default when the query itself
does not contain a script hint.

Constraint: Keep the patch small and mergeable without redesigning per-user preference storage
Rejected: Add per-user backend language state | larger scope than the validated bug and unclear maintainer preference
Rejected: Persist preferred language in config | still shares mutable state across clients of the same instance
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If locale-aware provider routing is reintroduced later, scope it to explicit config or request context instead of package-global state
Tested: go test ./web/backend/api ./pkg/tools -count=1; pnpm lint; pnpm build
Not-tested: Full make check; live multi-browser manual launcher run after the backend endpoint removal

web/backend/api/tools_test.go

@@ -9,7 +9,6 @@ import (
 	"testing"
 
 	"github.com/sipeed/picoclaw/pkg/config"
-	picotools "github.com/sipeed/picoclaw/pkg/tools"
 )
 
 func TestHandleListTools(t *testing.T) {
@@ -517,22 +516,12 @@ func TestResolveCurrentWebSearchProvider_FallsBackWhenProviderIsUnknown(t *testi
 	}
 }
 
-func TestResolveCurrentWebSearchProvider_UsesPreferredLanguageForSogouAndDuckDuckGo(t *testing.T) {
+func TestResolveCurrentWebSearchProvider_PrefersStableDefaultForSogouAndDuckDuckGo(t *testing.T) {
 	cfg := config.DefaultConfig()
 	cfg.Tools.Web.Provider = "auto"
 	cfg.Tools.Web.Sogou.Enabled = true
 	cfg.Tools.Web.DuckDuckGo.Enabled = true
 
-	picotools.SetPreferredWebSearchLanguage("en")
-	t.Cleanup(func() {
-		picotools.SetPreferredWebSearchLanguage("")
-	})
-
-	if got := resolveCurrentWebSearchProvider(cfg); got != "duckduckgo" {
-		t.Fatalf("resolveCurrentWebSearchProvider() = %q, want duckduckgo", got)
-	}
-
-	picotools.SetPreferredWebSearchLanguage("zh")
 	if got := resolveCurrentWebSearchProvider(cfg); got != "sogou" {
 		t.Fatalf("resolveCurrentWebSearchProvider() = %q, want sogou", got)
 	}