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/frontend/src/i18n/index.ts

@@ -7,8 +7,6 @@ import i18n from "i18next"
 import LanguageDetector from "i18next-browser-languagedetector"
 import { initReactI18next } from "react-i18next"
 
-import { launcherFetch } from "@/api/http"
-
 import en from "./locales/en.json"
 import zh from "./locales/zh.json"
 
@@ -46,14 +44,6 @@ i18n.on("languageChanged", (lng) => {
   } else {
     dayjs.locale("en")
   }
-
-  void launcherFetch("/api/ui/language", {
-    method: "POST",
-    headers: { "Content-Type": "application/json" },
-    body: JSON.stringify({ language: lng }),
-  }).catch(() => {
-    // Keep UI language changes responsive even if backend sync fails.
-  })
 })
 
 export default i18n