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>

DESIGN.md

@@ -225,7 +225,8 @@ Used inside pages that need content partitioning without a URL change (e.g. Enga
 - **`.tab-underline-active`**: `text-primary border-primary` — 2px primary underline, primary text.
 - **Count pill** (optional, e.g. simulation count): `.tab-count-pill` — `rounded-pill bg-cloud text-graphite text-[11px] px-xs py-0 font-mono`. `rounded-pill` is the documented exception.
 - **Active count pill**: `.tab-count-pill-active` — `bg-primary-soft text-primary`.
-- ARIA: `role="tablist"` on container; `role="tab"` + `aria-selected` on each button.
+- **Disabled variant**: `.tab-underline-disabled` — `text-graphite opacity-50 cursor-not-allowed`. Applied when `disabled?: boolean` is set on a `TabItem`. Overrides `.tab-underline` hover. ARIA: `aria-disabled="true"` + native `disabled` attribute. Arrow-key navigation skips disabled items.
+- ARIA: `role="tablist"` on container; `role="tab"` + `aria-selected` + `aria-controls="tabpanel-{id}"` on each button.
 
 ### Data Tables