feat(tickets): fenêtre dédiée de gestion des tickets — liens via popup + assistant IA flottant (#17)

Lot C du sprint UI rework : fenêtre dédiée de gestion des tickets,
appuyée sur la primitive FloatingWindow (#16) et la popup TicketPicker (#18).

- TicketDetail : gestion des liens de ticket via la popup TicketPicker,
  assistant IA en fenêtre flottante
- FloatingWindow : ajustements pour l'usage fenêtre dédiée

Tests : tsc --noEmit clean, suite complète 530/530, suites tickets +
FloatingWindow 50/50.

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

frontend/src/shared/ui/FloatingWindow.tsx

@@ -34,6 +34,13 @@ const SIZE_CLASS: Record<FloatingWindowSize, string> = {
 const FOCUSABLE =
   'a[href],button:not([disabled]),input:not([disabled]),select:not([disabled]),textarea:not([disabled]),[tabindex]:not([tabindex="-1"])';
 
+/**
+ * Stack of currently-mounted windows (most-recent last). Only the topmost window
+ * reacts to Escape and traps Tab, so a window opened over another (e.g. a ticket
+ * detail over the tickets list) doesn't close/steal from the one beneath it.
+ */
+const windowStack: object[] = [];
+
 export interface FloatingWindowProps {
   /** Whether the window is shown. Rendered as `null` when closed. */
   open: boolean;
@@ -76,11 +83,17 @@ function FloatingWindowBody({
   useEffect(() => {
     const previouslyFocused = document.activeElement as HTMLElement | null;
     const node = dialogRef.current;
+    // Register on the window stack so only the topmost handles keys.
+    const token = {};
+    windowStack.push(token);
+    const isTopmost = () => windowStack[windowStack.length - 1] === token;
     // Move focus into the window (first focusable, else the dialog itself).
     const first = node?.querySelector<HTMLElement>(FOCUSABLE);
     (first ?? node)?.focus();
 
     function onKeyDown(e: KeyboardEvent) {
+      // Ignore keys while a later window is stacked on top of this one.
+      if (!isTopmost()) return;
       if (e.key === "Escape") {
         e.preventDefault();
         onClose();
@@ -111,6 +124,8 @@ function FloatingWindowBody({
     document.addEventListener("keydown", onKeyDown);
     return () => {
       document.removeEventListener("keydown", onKeyDown);
+      const i = windowStack.indexOf(token);
+      if (i >= 0) windowStack.splice(i, 1);
       previouslyFocused?.focus?.();
     };
   }, [onClose]);