fix(vite): basePath handling for static assets (freshframework#3394)

closes freshframework#3391 

---
Generated PR message:

Fix base path handling for assets in plugin-vite

Fixes issue where Fresh apps mounted at non-root paths (e.g., /ui) in
frameworks like Hono would fail to load assets correctly.

  Problem

When mounting a Fresh app with app.mount('/ui', UI.fetch), the HTML
would load but all assets (CSS, JS, images) would return 404 because
asset paths
were hardcoded to start with /assets/ instead of respecting the Vite
base configuration.

  Solution

- Modified packages/plugin-vite/src/plugins/server_entry.ts to read
Vite's base config and apply it to asset paths
- Added getAssetPath() helper function to construct proper asset URLs
with base path
  - Updated both CSS and asset file registration to use the base path

  Testing

Added comprehensive test case "vite build - base path asset handling"
that:
  - Builds demo with custom base path /my-app/
  - Verifies generated server.js contains correctly prefixed asset paths
  - Follows existing test patterns and utilities

  Usage

  Users can now properly mount Fresh apps by configuring both:
```ts
  // vite.config.ts
  export default defineConfig({
    base: "/ui/",
    plugins: [fresh()],
  });

  // main.ts  
  const app = new App({ basePath: "/ui" });

  // hono-app.ts
  app.mount('/ui', UI.fetch); // Assets now work correctly
```

  Backward compatible - no changes needed for apps mounted at root path.

Author: fry69

packages/plugin-vite/src/plugins/server_entry.ts

@@ -16,9 +16,19 @@ export function serverEntryPlugin(
   let serverOutDir = "";
   let clientOutDir = "";
   let root = "";
+  let basePath = "";
 
   let isDev = false;
 
+  const getAssetPath = (id: string): string => {
+    if (basePath === "/") {
+      return `/${id}`;
+    }
+    // Ensure basePath ends with / and construct the path manually to avoid platform-specific path issues
+    const normalizedBase = basePath.endsWith("/") ? basePath : basePath + "/";
+    return normalizedBase + id;
+  };
+
   return {
     name: "fresh:server_entry",
     applyToEnvironment(env) {
@@ -29,6 +39,10 @@ export function serverEntryPlugin(
     },
     configResolved(config) {
       root = config.root;
+      basePath = config.base || "/";
+      if (basePath !== "/" && !basePath.endsWith("/")) {
+        basePath += "/";
+      }
       serverEntry = pathWithRoot(options.serverEntry, config.root);
       serverOutDir = pathWithRoot(
         config.environments.ssr.build.outDir,
@@ -98,7 +112,7 @@ if (import.meta.hot) import.meta.hot.accept();`;
               staticFiles.push({
                 filePath: path.join(serverOutDir, id),
                 hash: null,
-                pathname: `/${id}`,
+                pathname: getAssetPath(id),
               });
             }
           }
@@ -110,7 +124,7 @@ if (import.meta.hot) import.meta.hot.accept();`;
               staticFiles.push({
                 filePath: path.join(serverOutDir, id),
                 hash: null,
-                pathname: `/${id}`,
+                pathname: getAssetPath(id),
               });
             }
           }

packages/plugin-vite/tests/build_test.ts

@@ -447,3 +447,20 @@ Deno.test({
   sanitizeOps: false,
   sanitizeResources: false,
 });
+
+Deno.test({
+  name: "vite build - base path asset handling",
+  fn: async () => {
+    await using res = await buildVite(DEMO_DIR, { base: "/my-app/" });
+
+    // Read the generated server.js to check asset paths
+    const serverJs = await Deno.readTextFile(
+      path.join(res.tmp, "_fresh", "server.js"),
+    );
+
+    // Asset paths should include the base path /my-app/
+    expect(serverJs).toContain('"/my-app/assets/');
+  },
+  sanitizeOps: false,
+  sanitizeResources: false,
+});

packages/plugin-vite/tests/test_utils.ts

@@ -131,7 +131,10 @@ export async function withDevServer(
   await launchDevServer(tmp.dir, fn, env);
 }
 
-export async function buildVite(fixtureDir: string) {
+export async function buildVite(
+  fixtureDir: string,
+  options?: { base?: string },
+) {
   const tmp = await withTmpDir({
     dir: path.join(import.meta.dirname!, ".."),
     prefix: "tmp_vite_",
@@ -140,6 +143,7 @@ export async function buildVite(fixtureDir: string) {
   const builder = await createBuilder({
     logLevel: "error",
     root: fixtureDir,
+    base: options?.base,
     build: {
       emptyOutDir: true,
     },