Add ESLint rule to catch broken internal links

.eslintignore

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+node_modules/
+build/
+.docusaurus/

.eslintrc.js

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+module.exports = {
+  parserOptions: {
+    ecmaVersion: 2022,
+    sourceType: "module",
+    ecmaFeatures: { jsx: true },
+  },
+  plugins: ["react"],
+  settings: {
+    react: { version: "detect" },
+  },
+  rules: {
+    "no-bad-internal-links": "error",
+  },
+};

.github/workflows/lint.yml

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+name: Lint
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Lint
+        run: npm run lint

eslint-rules/no-bad-internal-links.js

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+/**
+ * ESLint rule: no-bad-internal-links
+ *
+ * Enforces correct internal link format in JSX:
+ * - No leading/trailing whitespace in any URL
+ * - Internal links must start with '/'
+ * - Internal links must not use .html extension
+ * - Internal links must end with '/'
+ */
+module.exports = {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "Enforce correct internal link format in JSX (absolute paths, no .html, trailing slash)",
+    },
+    messages: {
+      whitespace:
+        "Link has leading or trailing whitespace: '{{value}}'. Remove the extra spaces.",
+      noLeadingSlash:
+        "Internal link '{{value}}' should start with '/'. Did you mean '/{{suggested}}'?",
+      htmlExtension:
+        "Internal link '{{value}}' should not use .html extension. Did you mean '{{suggested}}'?",
+      noTrailingSlash:
+        "Internal link '{{value}}' should end with '/'. Did you mean '{{value}}/'?",
+    },
+  },
+  create(context) {
+    return {
+      JSXAttribute(node) {
+        const attrName = node.name && node.name.name;
+        if (attrName !== "href" && attrName !== "to") return;
+
+        // Only handle string literals, skip expressions like {`/path/${var}`}
+        if (
+          !node.value ||
+          node.value.type !== "Literal" ||
+          typeof node.value.value !== "string"
+        ) {
+          return;
+        }
+
+        const value = node.value.value;
+
+        // Check for leading/trailing whitespace (applies to all URLs)
+        if (value !== value.trim()) {
+          context.report({ node, messageId: "whitespace", data: { value } });
+          return;
+        }
+
+        // Skip external URLs, anchors, and protocol-relative URLs
+        if (
+          value.startsWith("http://") ||
+          value.startsWith("https://") ||
+          value.startsWith("mailto:") ||
+          value.startsWith("tel:") ||
+          value.startsWith("#") ||
+          value.startsWith("//")
+        ) {
+          return;
+        }
+
+        // Skip empty strings
+        if (value === "") return;
+
+        // --- Internal link checks ---
+
+        // Must start with /
+        if (!value.startsWith("/")) {
+          const stripped = value.replace(/\.html?$/, "");
+          const suggested = stripped.endsWith("/") ? stripped : stripped + "/";
+          context.report({
+            node,
+            messageId: "noLeadingSlash",
+            data: { value, suggested },
+          });
+          return;
+        }
+
+        // Must not end with .html
+        if (/\.html?$/.test(value)) {
+          const suggested = value.replace(/\.html?$/, "/");
+          context.report({
+            node,
+            messageId: "htmlExtension",
+            data: { value, suggested },
+          });
+          return;
+        }
+
+        // Must end with / (except paths with anchors or query strings)
+        if (
+          !value.endsWith("/") &&
+          !value.includes("#") &&
+          !value.includes("?")
+        ) {
+          context.report({
+            node,
+            messageId: "noTrailingSlash",
+            data: { value },
+          });
+        }
+      },
+    };
+  },
+};