refactor(desktop): harden workspace shell foundation

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

plans/2026-03-31-hac-26-workspace-state-foundation.md

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+# HAC-26 Workspace State Foundation
+
+Date: 2026-03-31 UTC
+Owner: Senior Frontend Engineer
+Issue: HAC-26
+Scope: `apps/desktop` frontend foundation only
+
+## Context
+
+- `apps/desktop/src/App.tsx` already owns the visible desktop-v1 shell, but it still contains repeated inline logic for key filtering, selected-key fallback, live key replacement, and paged append handling.
+- These state transitions are part of the browsing and inspector foundation, not one-off UI copy, and upcoming desktop-v1 slices will need them again.
+- The PM comment on `HAC-26` asked for the highest reusable foundation return, not a new visible feature.
+
+## Decision
+
+- Do not expand product scope.
+- Extract reusable key-workspace state helpers from `App.tsx` into `src/lib`.
+- Cover the extracted state transitions with focused Vitest cases so future desktop-v1 changes can reuse the same behavior safely.
+
+## Planned Implementation
+
+1. Create a shared frontend utility for:
+   - filtering visible keys by search text
+   - resolving selected key fallback when visible results change
+   - replacing a key record after live metadata/value refresh
+   - appending paged browse results without duplicate rows
+2. Update `App.tsx` to consume the shared utility instead of inline logic.
+3. Add Vitest coverage for zero-result fallback, selection resolution, replacement merge behavior, and append deduplication.
+4. Re-run `pnpm --filter @redis-gui/desktop test` and `pnpm run desktop:build`.
+
+## Acceptance
+
+- `App.tsx` no longer owns the reusable key-workspace list and selection transitions inline.
+- The extracted helper is covered by focused tests.
+- Existing desktop frontend tests and production build remain green.
+- No backend contract or visible feature scope changes are introduced.