Add linting to frontend (#4341)

# Description of Changes
There's no current linter running over our TypeScript code, which means
we've got a bunch of dead code and other code smells around with nothing
notifying us. This PR adds ESLint with the typescript-eslint plugin and
enables the recommended settings as a starting point for us.

I've disabled all of the failing rules for the scope of this PR, just to
get linting running without causing a massive diff. I'll follow up with
future PRs that enable the failing rules one by one.

Also updates our version of TypeScript, which introduces a new type
error in the code (which I've had to fix)

frontend/package.json

@@ -40,6 +40,7 @@
     "predev": "npm run generate-icons",
     "dev": "npx tsc --noEmit && vite",
     "prebuild": "npm run generate-icons",
+    "lint": "npx eslint",
     "build": "npx tsc --noEmit && vite build",
     "preview": "vite preview",
     "typecheck": "tsc --noEmit",
@@ -72,6 +73,7 @@
     ]
   },
   "devDependencies": {
+    "@eslint/js": "^9.34.0",
     "@iconify-json/material-symbols": "^1.2.33",
     "@iconify/utils": "^3.0.1",
     "@playwright/test": "^1.40.0",
@@ -80,6 +82,7 @@
     "@types/react-dom": "^19.1.5",
     "@vitejs/plugin-react": "^4.5.0",
     "@vitest/coverage-v8": "^1.0.0",
+    "eslint": "^9.34.0",
     "jsdom": "^23.0.0",
     "license-checker": "^25.0.1",
     "madge": "^8.0.0",
@@ -87,7 +90,8 @@
     "postcss-cli": "^11.0.1",
     "postcss-preset-mantine": "^1.17.0",
     "postcss-simple-vars": "^7.0.1",
-    "typescript": "^5.8.3",
+    "typescript": "^5.9.2",
+    "typescript-eslint": "^8.42.0",
     "vite": "^6.3.5",
     "vitest": "^1.0.0"
   }