feat(desktop): unify frontend error notices

Co-Authored-By: Paperclip <noreply@paperclip.ing>

plans/2026-03-28-hac-17-foundation-hardening.md

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+# HAC-17 Frontend Foundation Hardening
+
+Date: 2026-03-28
+Owner: Senior Frontend Engineer
+Issue: HAC-17
+
+## Goal
+
+Harden the desktop frontend foundation for upcoming iterations without changing backend contracts or expanding product scope.
+
+## Selected Scope
+
+- extract a shared frontend error mapping layer that translates stable backend error codes into consistent operator-facing copy
+- route connection test, key browse, value read, string save, TTL update, inspector refresh, and command execution failures through the shared mapper
+- add the minimum test baseline around the new error translation logic so future feature work can extend it safely
+- update shared run and acceptance docs so QA can verify the new error behavior
+
+## Non-Goals
+
+- no backend contract change
+- no new Redis capability surface
+- no visual redesign outside the existing desktop shell language
+- no delete execution or broader write support
+
+## Verification Plan
+
+- `pnpm --filter @redis-gui/desktop test`
+- `pnpm run desktop:build`
+
+## Notes
+
+- The existing shell already exposes stable backend error codes from `redis-core`; the current gap is inconsistent UI handling rather than missing backend semantics.