fix: eslint findings

Remove dead code and unused imports, drop useless assignments, and tune
rules in eslint.config.js (underscore-prefixed unused names, intentional BOM/NBSP,
React Compiler rules from eslint-plugin-react-hooks 7 as warnings).

eslint.config.js

@@ -10,6 +10,35 @@ export default tseslint.config(
   js.configs.recommended,
   tseslint.configs.recommended,
   reactHooks.configs.flat.recommended,
+  {
+    rules: {
+      // Leading underscore marks an intentionally unused binding (omitting a key via rest, unused
+      // override parameters).
+      "@typescript-eslint/no-unused-vars": [
+        "error",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_", caughtErrorsIgnorePattern: "^_", ignoreRestSiblings: true },
+      ],
+      // The BOM and NBSP are written literally on purpose in the CSV exporters and the text sanitiser.
+      "no-irregular-whitespace": ["error", { skipStrings: true, skipTemplates: true, skipRegExps: true }],
+    },
+  },
+  {
+    // The React Compiler rules shipped in eslint-plugin-react-hooks 7 flag long-standing patterns
+    // here (load data on mount, "latest value" refs written during render). They are legitimate
+    // design feedback but not bugs, and rewriting them blind risks behaviour changes in an
+    // effect-heavy app, so they stay visible as warnings until each site is reworked.
+    files: ["src/**/*.{ts,tsx}"],
+    rules: {
+      "react-hooks/set-state-in-effect": "warn",
+      "react-hooks/refs": "warn",
+      "react-hooks/immutability": "warn",
+    },
+  },
+  {
+    // Test helper that reads react-pdf's untyped internal layout data.
+    files: ["src/pdf/testing/**/*.ts"],
+    rules: { "@typescript-eslint/no-explicit-any": "warn" },
+  },
   {
     files: ["src/**/*.{ts,tsx}"],
     languageOptions: { globals: globals.browser },