feat(frontend): SimulationFormPage 3-tab layout (Red Team / SOC / Tâche C2)

- Tabs: disabled per-item (aria-disabled, native disabled, tab-underline-disabled,
  arrow-key skip); aria-controls wired
- useHashTab: optional validIds fallback to defaultId
- SimulationFormPage: 3-tab refactor; SOC tab gated on review_required|done;
  C2 tab disabled in create mode; contextual sticky bar hidden on C2 tab;
  safeTab guard against stale hash; SOC-blocked banner removed
- fr.json: simulation.form.tab.{redTeam,soc,c2} keys
- DESIGN.md: disabled tab variant + aria-controls documented
- Tests: 253 passing (5 new Tabs disabled tests, SimulationFormPage rewritten
  for tab layout)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/Tabs.tsx

@@ -4,6 +4,7 @@ interface TabItem {
   id: string;
   label: string;
   count?: number;
+  disabled?: boolean;
 }
 
 interface TabsProps {
@@ -13,13 +14,23 @@ interface TabsProps {
 }
 
 export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
+  function nextEnabled(from: number, direction: 1 | -1): string {
+    const len = items.length;
+    let i = (from + direction + len) % len;
+    while (i !== from) {
+      if (!items[i].disabled) return items[i].id;
+      i = (i + direction + len) % len;
+    }
+    return items[from].id;
+  }
+
   function handleKeyDown(e: KeyboardEvent<HTMLButtonElement>, index: number) {
     if (e.key === 'ArrowRight') {
       e.preventDefault();
-      onChange(items[(index + 1) % items.length].id);
+      onChange(nextEnabled(index, 1));
     } else if (e.key === 'ArrowLeft') {
       e.preventDefault();
-      onChange(items[(index - 1 + items.length) % items.length].id);
+      onChange(nextEnabled(index, -1));
     }
   }
 
@@ -30,6 +41,7 @@ export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
     >
       {items.map((item, index) => {
         const isActive = item.id === activeId;
+        const isDisabled = item.disabled ?? false;
         return (
           <button
             key={item.id}
@@ -37,10 +49,12 @@ export function Tabs({ items, activeId, onChange }: TabsProps): JSX.Element {
             role="tab"
             aria-selected={isActive}
             aria-controls={`tabpanel-${item.id}`}
+            aria-disabled={isDisabled || undefined}
+            disabled={isDisabled}
             type="button"
-            onClick={() => onChange(item.id)}
+            onClick={() => { if (!isDisabled) onChange(item.id); }}
             onKeyDown={(e) => handleKeyDown(e, index)}
-            className={`tab-underline${isActive ? ' tab-underline-active' : ''} pb-xs`}
+            className={`tab-underline${isActive ? ' tab-underline-active' : ''}${isDisabled ? ' tab-underline-disabled' : ''} pb-xs`}
           >
             {item.label}
             {item.count !== undefined ? (