Replace sonner toasts with one top-centre notification stack

The app said things in two places: sonner's bottom-right toasts and the
custom top-centre chrome. This is the second of those, generalised — a
module-level store anyone can call, drawn as a stack of frosted cards that go
on their own unless raised as persistent, with room for buttons and for a
message longer than a line.

useCustomToast keeps its signatures, so the forty-odd call sites behind it and
handleError are untouched. The component and its store are byte-identical with
the portal's copy and checked by `make design-check`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -48,7 +48,6 @@
     "lucide-react": "^0.562.0",
     "monaco-editor": "^0.56.0",
     "motion": "^13.0.0",
-    "next-themes": "^0.4.6",
     "radix-ui": "^1.6.7",
     "react": "^19.1.1",
     "react-dom": "^19.2.3",
@@ -56,7 +55,6 @@
     "react-grid-layout": "^2.2.4",
     "react-hook-form": "^7.68.0",
     "react-icons": "^5.5.0",
-    "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "tailwindcss": "^4.1.17",
     "uplot": "^1.6.32",