feat(desktop): harden database switcher range

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

plans/2026-03-31-hac-22-db-switcher-hardening.md

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+# HAC-22 DB Switcher Hardening
+
+Date: 2026-03-31 UTC
+Owner: Senior Frontend Engineer
+Issue: HAC-22
+Scope: `desktop-v1` user-visible frontend hardening only
+
+## Context
+
+- `apps/desktop` already ships a visible DB switcher, but the current UI only renders four fixed choices: `db0`, `db1`, `db2`, and `db5`.
+- Current product scope already includes one active standalone Redis connection, explicit DB context, and DB-scoped browse / inspect / command flows.
+- The hardcoded four-item list is weaker than the current product boundary and makes shell-level verification less representative for operators who need to move across common DB indexes.
+
+## Decision
+
+- Do not expand product scope or alter backend contracts.
+- Keep the existing rail placement and interaction model for the DB switcher.
+- Widen the visible DB switcher to a predictable default operator range and ensure the currently active DB remains selectable even when it falls outside the default range.
+
+## Implemented Slice
+
+1. Added a shared frontend helper that returns DB labels for the switcher.
+2. Defaulted the visible range to `db0` through `db15`.
+3. Preserved the currently active DB when it falls outside that range and kept the options numerically ordered.
+4. Covered the helper with focused Vitest cases.
+
+## Acceptance
+
+- The visible DB switcher no longer depends on a four-item hardcoded list.
+- Operators can select common DB indexes from `db0` through `db15` without editing connection drafts.
+- An active DB outside the default range still appears in the switcher and stays selectable.
+- `pnpm --filter @redis-gui/desktop test` and `pnpm run desktop:build` remain green.