feat(layout): supporte la création de layouts plugins (customPluginLayout)

Étend le flux de création de layout backend (DTO, usecases, store) et le
frontend (sélecteur, adaptateurs, LayoutTabs/LayoutGrid) pour permettre
d'ouvrir un layout déclaré par un plugin installé (ex. Android Health),
sans passer par le message bloquant "extension backend pas encore livrée".

Ticket #141 — QA vert.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/plugins/PluginLayoutSelectorSection.tsx

@@ -6,15 +6,8 @@
  * plugins never appear — carnet §1.3: only `enabled` plugins are loaded, so
  * there is nothing to filter here beyond what the registry already omits).
  *
- * Presentational only; not yet mounted in a layout-creation flow. The
- * existing "layout" concept in this codebase (`LayoutTabs`, `LayoutKind`) is
- * a whole-tab kind (`"terminal" | "gitGraph"`) picked via a fixed two-item
- * dropdown, backed by a `create(name, kind)` Tauri command that only knows
- * those two kinds. Wiring an actual "create a plugin layout" action needs a
- * backend `LayoutKind`/`create_layout` extension (carnet §10 flags F4 as
- * "DevFrontend + DevBackend si ajustement DTO layout") — this component is
- * the frontend half, ready to drop into that flow once the DTO lands; see the
- * F4 delivery report's open point.
+ * Presentational only: the caller owns whether selecting a contribution creates
+ * a named plugin layout, replaces an unavailable cell, or just previews it.
  */
 
 import type { PluginLayoutContribution } from "@/domain";