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>

bun.lock

@@ -42,7 +42,6 @@
         "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",
@@ -50,7 +49,6 @@
         "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",
@@ -787,8 +785,6 @@
 
     "neo-async": ["neo-async@2.6.2", "", {}, "sha512-Yd3UES5mWCSqR+qNT93S3UoYUkqAZ9lLg8a7g9rimsWmYGK8cVToA4/sF3RrshdyV3sAGMXVUmpMYOw+dLpOuw=="],
 
-    "next-themes": ["next-themes@0.4.6", "", { "peerDependencies": { "react": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc", "react-dom": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc" } }, "sha512-pZvgD5L0IEvX5/9GWyHMf3m8BKiVQwsCMHfoFosXtXBMnaS0ZnIJ9ST4b4NqLVKDEm8QBxoNNGNaBv2JNF6XNA=="],
-
     "node-fetch-native": ["node-fetch-native@1.6.7", "", {}, "sha512-g9yhqoedzIUm0nTnTqAQvueMPVOuIY16bqgAJJC8XOOubYFNwz6IER9qs0Gq2Xd0+CecCKFjtdDTMA4u4xG06Q=="],
 
     "node-releases": ["node-releases@2.0.27", "", {}, "sha512-nmh3lCkYZ3grZvqcCH+fjmQ7X+H0OeZgP40OierEaAptX4XofMh5kwNbWh7lBduUzCcV/8kZ+NDLCwm2iorIlA=="],
@@ -873,8 +869,6 @@
 
     "seroval-plugins": ["seroval-plugins@1.4.2", "", { "peerDependencies": { "seroval": "^1.0" } }, "sha512-X7p4MEDTi+60o2sXZ4bnDBhgsUYDSkQEvzYZuJyFqWg9jcoPsHts5nrg5O956py2wyt28lUrBxk0M0/wU8URpA=="],
 
-    "sonner": ["sonner@2.0.7", "", { "peerDependencies": { "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc", "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc" } }, "sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w=="],
-
     "source-map": ["source-map@0.6.1", "", {}, "sha512-UjgapumWlbMhkBgzT7Ykc5YXUT46F0iKu8SGXq0bcwP5dz/h0Plj6enJqjz1Zbq2l5WaqYnrVbwWOWMyF3F47g=="],
 
     "source-map-js": ["source-map-js@1.2.1", "", {}, "sha512-UXWMKhLOwVKb728IUtQPXxfYU+usdybtUrK/8uGE8CQMvrhOpwvzDBwj0QhSL7MQc7vIsISBG8VQ8+IDQxpfQA=="],