feat: implement PR #20 review feedback

- Add eslint-plugin-react-hooks dependency to fix ESLint errors
- Fix unused sentimentThreshold variable in settings route
- Add comprehensive dark mode accessibility tests as requested
- Implement custom error classes for better error handling
- Create centralized error handling system with proper typing
- Add dark mode contrast and focus indicator tests
- Extend accessibility test coverage for theme switching

biome.json

@@ -0,0 +1,60 @@
+{
+  "$schema": "https://biomejs.dev/schemas/2.0.6/schema.json",
+  "linter": {
+    "enabled": true,
+    "rules": {
+      "recommended": true,
+      "correctness": {
+        "noUnusedVariables": "error",
+        "noUnusedImports": "error"
+      },
+      "style": {
+        "useConst": "error",
+        "useTemplate": "error"
+      },
+      "suspicious": {
+        "noExplicitAny": "warn",
+        "noArrayIndexKey": "warn"
+      },
+      "complexity": {
+        "noForEach": "off"
+      }
+    }
+  },
+  "formatter": {
+    "enabled": true,
+    "formatWithErrors": false,
+    "indentStyle": "space",
+    "indentWidth": 2,
+    "lineEnding": "lf",
+    "lineWidth": 80
+  },
+  "javascript": {
+    "formatter": {
+      "jsxQuoteStyle": "double",
+      "quoteProperties": "asNeeded",
+      "trailingCommas": "es5",
+      "semicolons": "always",
+      "arrowParentheses": "always",
+      "bracketSpacing": true,
+      "bracketSameLine": false,
+      "quoteStyle": "double"
+    }
+  },
+  "json": {
+    "formatter": {
+      "enabled": true
+    }
+  },
+  "files": {
+    "includes": [
+      "app/**",
+      "lib/**",
+      "components/**",
+      "*.ts",
+      "*.tsx",
+      "*.js",
+      "*.jsx"
+    ]
+  }
+}