Replace Sonner with custom Sera UI-inspired toast system (#296)

* refactor(ui): replace Sonner with custom Sera UI-inspired toast system

Replace the Sonner toast library with a custom implementation inspired by
Sera UI's glassmorphism design. The new system uses an external store pattern
with useSyncExternalStore for React integration, keeping the same
toast.success()/error()/warning() API so all 172 call sites required only
an import path change.

Key changes:
- New toast-store.ts: singleton store with identical API to Sonner
- New toast.tsx: Sera UI-faithful Notification component with Framer Motion
  animations, frosted glass (backdrop-blur-xl), type gradient overlays,
  animated progress bar (green→blue→sky gradient), and hover:scale-105
- Removed sonner and next-themes dependencies
- Rewired all 19 consumer files to import from the new store

* fix(ui): resolve ESLint errors in toast system

- Use const for listeners Set (prefer-const)
- Initialize startRef with 0 instead of Date.now() to satisfy
  react-hooks/purity rule, set actual value inside useEffect

frontend/package-lock.json

@@ -38,14 +38,12 @@
         "lucide-react": "^1.6.0",
         "monaco-editor": "^0.55.1",
         "motion": "^12.38.0",
-        "next-themes": "^0.4.6",
         "radix-ui": "^1.4.3",
         "react": "^19.2.0",
         "react-dom": "^19.2.0",
         "react-is": "^19.2.4",
         "react-use-measure": "^2.1.7",
         "recharts": "^3.8.1",
-        "sonner": "^2.0.7",
         "tailwind-merge": "^3.5.0",
         "tailwindcss-animate": "^1.0.7",
         "xterm-addon-search": "^0.13.0",
@@ -4896,16 +4894,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "node_modules/next-themes": {
-      "version": "0.4.6",
-      "resolved": "https://registry.npmjs.org/next-themes/-/next-themes-0.4.6.tgz",
-      "integrity": "sha512-pZvgD5L0IEvX5/9GWyHMf3m8BKiVQwsCMHfoFosXtXBMnaS0ZnIJ9ST4b4NqLVKDEm8QBxoNNGNaBv2JNF6XNA==",
-      "license": "MIT",
-      "peerDependencies": {
-        "react": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc",
-        "react-dom": "^16.8 || ^17 || ^18 || ^19 || ^19.0.0-rc"
-      }
-    },
     "node_modules/node-releases": {
       "version": "2.0.27",
       "resolved": "https://registry.npmjs.org/node-releases/-/node-releases-2.0.27.tgz",
@@ -5458,16 +5446,6 @@
         "node": ">=8"
       }
     },
-    "node_modules/sonner": {
-      "version": "2.0.7",
-      "resolved": "https://registry.npmjs.org/sonner/-/sonner-2.0.7.tgz",
-      "integrity": "sha512-W6ZN4p58k8aDKA4XPcx2hpIQXBRAgyiWVkYhT7CvK6D3iAu7xjvVyhQHg2/iaKJZ1XVJ4r7XuwGL+WGEK37i9w==",
-      "license": "MIT",
-      "peerDependencies": {
-        "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc",
-        "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
-      }
-    },
     "node_modules/source-map-js": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",