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
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