fix(frontend): SOC C2 tab RBAC gate + safeTab sync + disabled tab contrast (review polish)

- c2TabDisabled: add isSoc condition — SOC role has no access to /c2/tasks (backend 403)
- safeTab sync: useEffect before early returns using detail.data?.status; syncs
  window.location.hash when hash points to a now-disabled tab after status transition
- create-mode submit button: type="submit" form="" → type="button" (form association fix)
- .tab-underline-disabled: text-graphite opacity-50 → text-steel (consistent with
  btn-outline:disabled; better perceptual contrast)
- tests: 254 passing (+ SOC × C2 tab disabled assertion)

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

DESIGN.md

@@ -225,7 +225,7 @@ 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`.
-- **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.
+- **Disabled variant**: `.tab-underline-disabled` — `text-steel cursor-not-allowed`. Uses the same `text-steel` token as `btn-outline:disabled` for system-wide consistency. 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