fix(site): derive base from GITHUB_REPOSITORY and fix 404 redirect target

Addresses Copilot review feedback on PR #11.

vite.config.ts:
  Production `base` is no longer hardcoded to /coder-skill-scanner/.
  Resolves via:
    1. VITE_BASE_PATH env (explicit override)
    2. GITHUB_REPOSITORY (CI default; forks Just Work with zero config)
    3. / (apex / local-build fallback)
  Dev keeps / so the local Vite server and report proxy work without
  env juggling.

public/404.html + new rewrite-public-base-url Vite plugin:
  The GitHub Pages SPA-fallback page redirected to /?p=... (apex),
  which bounced users off the project page and onto the github.io
  apex instead of bringing them back into the SPA. Use %BASE_URL% as
  the redirect prefix and substitute it at build time via a tiny
  plugin (Vite handles the substitution for index.html but copies
  public/ verbatim).

Verified with four build runs (GITHUB_REPOSITORY=coder/coder-skill-scanner,
GITHUB_REPOSITORY=someone-else/my-scanner, VITE_BASE_PATH override, no
env). In every case the bases emitted in dist/index.html and the
target string in dist/404.html match.

Author: DevelopmentCats

site/public/404.html

@@ -5,9 +5,14 @@
     <title>Coder Skill Scanner</title>
     <script>
       // GitHub Pages SPA fallback: capture the original path in a query
-      // parameter and bounce to index.html, which restores it before React
-      // Router boots. See site/src/main.tsx for the restore step.
-      var target = "/?p=" + encodeURIComponent(
+      // parameter and bounce to the SPA index, which restores it before
+      // React Router boots. See site/src/main.tsx for the restore step.
+      //
+      // `%BASE_URL%` is substituted at build time by the
+      // `rewrite-public-base-url` Vite plugin (see site/vite.config.ts),
+      // so the bounce lands on the correct base prefix for both apex
+      // sites (`/`) and project pages (`/<repo>/`).
+      var target = "%BASE_URL%?p=" + encodeURIComponent(
         window.location.pathname + window.location.search
       ) + window.location.hash;
       window.location.replace(target);

site/vite.config.ts

@@ -1,5 +1,7 @@
-import { defineConfig, loadEnv } from "vite";
+import { defineConfig, loadEnv, type PluginOption } from "vite";
 import react from "@vitejs/plugin-react-swc";
+import fs from "node:fs";
+import path from "node:path";
 
 /**
  * The React app expects to fetch `latest.json`, `schema.json`, and
@@ -14,16 +16,62 @@ import react from "@vitejs/plugin-react-swc";
  */
 const REPORT_REGEX = /^\/(latest\.json|schema\.json|history\/.+\.json)$/;
 
+/**
+ * Resolve the production `base` path. Lookup priority:
+ *   1. `VITE_BASE_PATH` env (explicit override; useful for local prod
+ *      builds when you want to mimic a specific Pages deployment).
+ *   2. The repo name parsed from `GITHUB_REPOSITORY` (CI default; a fork
+ *      named `<owner>/<repo>` gets `/<repo>/` automatically with zero
+ *      config).
+ *   3. `/` (apex / local-build fallback).
+ *
+ * Always returns a value with leading and trailing slashes so Vite's
+ * own asset URL logic does not have to special-case the input.
+ */
+function resolveProductionBase(): string {
+  const explicit = process.env.VITE_BASE_PATH?.trim();
+  if (explicit) {
+    const prefixed = explicit.startsWith("/") ? explicit : `/${explicit}`;
+    return prefixed.endsWith("/") ? prefixed : `${prefixed}/`;
+  }
+  const repo = process.env.GITHUB_REPOSITORY?.split("/")[1]?.trim();
+  if (repo) return `/${repo}/`;
+  return "/";
+}
+
+/**
+ * Vite already substitutes `%BASE_URL%` in `index.html`, but it copies
+ * everything under `public/` verbatim. This plugin runs after the build
+ * finishes and substitutes the same placeholder in `dist/404.html` so
+ * the GitHub Pages SPA-fallback redirect targets the right base prefix
+ * regardless of which fork is publishing.
+ */
+function rewritePublicBaseUrl(base: string): PluginOption {
+  return {
+    name: "rewrite-public-base-url",
+    apply: "build",
+    writeBundle() {
+      const targets = ["404.html"];
+      for (const name of targets) {
+        const file = path.resolve("dist", name);
+        if (!fs.existsSync(file)) continue;
+        const content = fs.readFileSync(file, "utf8");
+        fs.writeFileSync(file, content.replace(/%BASE_URL%/g, base));
+      }
+    },
+  };
+}
+
 export default defineConfig(({ mode, command }) => {
   const env = loadEnv(mode, process.cwd(), "");
   const upstream = env.VITE_REPORT_UPSTREAM || "http://localhost:8765";
+  const base = command === "build" ? resolveProductionBase() : "/";
 
   return {
-    // GitHub Pages serves this site at /coder-skill-scanner/. Production
-    // builds emit asset URLs relative to that base. Dev keeps the default
-    // `/` so the local Vite server and its report proxy keep working.
-    base: command === "build" ? "/coder-skill-scanner/" : "/",
-    plugins: [react()],
+    // See `resolveProductionBase` above. Dev keeps `/` so the local Vite
+    // server and its report proxy keep working without env juggling.
+    base,
+    plugins: [react(), rewritePublicBaseUrl(base)],
     server: {
       host: "0.0.0.0",
       port: 5173,