feat(operator): command palette, impersonation, incident, tweaks (O.8)

- CommandPalette + useCommandPalette: ⌘K opens a search-and-jump panel over
  real tenants/partners + fixture flags + nav + actions. Arrow keys + Enter
  navigate, Escape/backdrop close. Recents are intentionally omitted for now;
  add when there's something to recent over.
- Impersonation stub: useImpersonation + ImpersonationModal + ImpersonationBanner.
  Modal opens from tenant detail and from the palette. Banner stays at the top
  of the shell until exited. No real OBO token is minted — wiring OAuth Token
  Exchange is tracked as a follow-up.
- IncidentModal + useIncidentModal: opened from the Overview and Infrastructure
  incident banners, renders the mock INCIDENT data with metrics, timeline and
  draft composer.
- TweaksPanel + useTweaks: floating bottom-right panel for theme (dark/light),
  density (comfy/compact), env badge (prod/staging/dev). Saved to localStorage.
- Theme/density apply via [data-theme] + [data-density] overrides in
  tokens.css. Topbar env badge now reads from useTweaks instead of a prop.
- Layout wires ⌘K + ⌘[ at the document level and mounts the palette + modals
  + banner + tweaks panel once for all pages.

apps/operator/composables/useImpersonation.ts

@@ -0,0 +1,34 @@
+// Visual-only impersonation state. Real impersonation requires an OAuth
+// on-behalf-of grant or admin-impersonation token endpoint; this composable
+// just tracks whether the modal is open + which tenant we're "viewing" so the
+// red banner and topbar warning surface render correctly. Tracked as a
+// follow-up in OPERATOR-PLAN.md.
+
+import type { Tenant } from '~/types/tenant'
+
+const candidate = ref<Tenant | null>(null) // shown in the confirm modal
+const active = ref<Tenant | null>(null) // shown in the persistent banner
+const asUser = ref<string>('first-user@example.com')
+
+export const useImpersonation = () => ({
+  candidate,
+  active,
+  asUser,
+  open: (tenant: Tenant) => {
+    candidate.value = tenant
+  },
+  cancel: () => {
+    candidate.value = null
+  },
+  confirm: (reason: string, userLabel?: string) => {
+    if (!candidate.value) return
+    active.value = candidate.value
+    asUser.value = userLabel ?? asUser.value
+    candidate.value = null
+    // eslint-disable-next-line no-console
+    console.log('[impersonation] entered', { tenant: active.value.slug, reason, asUser: asUser.value })
+  },
+  exit: () => {
+    active.value = null
+  },
+})