feat(tickets): popup réutilisable de sélection de ticket (#18)

Ajoute TicketPicker, popup modale réutilisable de sélection de ticket,
socle du chemin critique du sprint UI rework (#17 et #19 en dépendent).

- TicketPicker.tsx : popup de recherche/sélection réutilisable
- useTicketSearch.ts : hook de recherche/filtrage extrait et partagé
- TicketFacetsBar.tsx : barre de facettes extraite de TicketsPanel
  pour réutilisation (listing principal + picker)
- shared/ui/zIndex.ts : échelle z-index centralisée
- TicketsPanel : consomme la barre de facettes extraite

Tests : TicketPicker.test.tsx 8/8, suite tickets 37/37, tsc --noEmit clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shared/index.ts

@@ -31,3 +31,6 @@ export type { ToolbarProps } from "./ui/Toolbar";
 
 export { Spinner } from "./ui/Spinner";
 export type { SpinnerProps } from "./ui/Spinner";
+
+export { zIndex } from "./ui/zIndex";
+export type { ZIndexLevel } from "./ui/zIndex";

frontend/src/shared/ui/zIndex.ts

@@ -0,0 +1,28 @@
+/**
+ * Centralized z-index scale for full-window chrome overlays (G2 — UI rework).
+ *
+ * In-cell veils/portals keep their historical low values (chrome 2–5, F3 z-20,
+ * write-portal z-4, resume z-5); this scale is for floating windows and menus
+ * mounted at the **chrome level** (ProjectsView/App), never inside
+ * LayoutGrid/LeafView (G5).
+ *
+ * ⚠️ PROVISIONAL (lot B / ticket #18): created here because lot A (ticket #16)
+ * introduces the canonical `shared/ui/zIndex.ts` (+ `FloatingWindow`) in
+ * parallel. The values below MUST match the frozen scale; at merge, keep a
+ * single module (drop this one if lot A's is richer). See the sprint scoping
+ * note `ui-rework-sprint-scoping-contracts`.
+ */
+export const zIndex = {
+  /** Dropdown menus / popovers anchored to a control. */
+  menuDropdown: 40,
+  /** A top-level floating window (movable panel, editor overlay). */
+  floatingWindow: 50,
+  /** A floating window opened from within another floating window (e.g. a
+   *  ticket picker launched from a sprint/link dialog). Sits above it. */
+  floatingWindowNested: 60,
+  /** Transient toasts, always on top. */
+  toast: 70,
+} as const;
+
+/** A level name in the {@link zIndex} scale. */
+export type ZIndexLevel = keyof typeof zIndex;