refactor(frontend): improve type safety and migrate to ESLint 9

Migrate the frontend to ESLint 9, add `eslint.config.mjs`, and replace various `any` types with more specific or safer alternatives across the codebase.

- Update `package.json` with new ESLint and TypeScript-ESLint dependencies
- Replace `any` with `Record<string, unknown>` or specific types in `TemplatePicker`, `Editor`, and `FileBrowser`
- Improve error handling in `TeamSettings` and `Templates` using type guards
- Add explicit type imports for Excalidraw elements in `Editor.tsx`
- Refactor `Modal.tsx` for cleaner conditional logic
- Add `check` script to `package.json` for type checking

frontend/package.json

@@ -6,8 +6,9 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc && vite build",
+    "check": "tsc --noEmit",
     "preview": "vite preview",
-    "lint": "eslint . --ext ts,tsx --report-unused-disable-directives --max-warnings 0",
+    "lint": "eslint .",
     "typecheck": "tsc --noEmit",
     "test": "vitest",
     "test:ui": "vitest --ui",
@@ -37,9 +38,12 @@
     "@types/react": "^18.3.18",
     "@types/react-dom": "^18.3.5",
     "@vitejs/plugin-react": "^4.3.0",
+    "eslint": "^9.0.0",
+    "eslint-plugin-react": "^7.37.0",
     "jsdom": "^25.0.0",
     "sass": "^1.81.0",
     "typescript": "^5.6.0",
+    "typescript-eslint": "^8.0.0",
     "vite": "^6.0.0",
     "vitest": "^3.0.0"
   }