fix frontend process.env, cleanup (#44)

- fix `process.env` in frontend
- remove redundant frontend command "build-trends"
- remove outdated frontend/README.md
- add documentation about skipping Vercel build cache

.github/CONTRIBUTING.md

@@ -7,7 +7,7 @@ To set up the project GitHub-Stats-Extended locally, run the following commands:
 ```bash
 ./vercel-preparation.sh
 pnpm install
-pnpm --filter frontend run build-trends
+pnpm --filter frontend run build
 ```
 
 The easiest way to run and test the project is to deploy it to Vercel as described in the [deployment guide](../docs/deploy.md).

.github/workflows/ci.yml

@@ -41,7 +41,7 @@ jobs:
           ./vercel-preparation.sh
 
       - name: Build frontend
-        run: pnpm --filter frontend run build-trends
+        run: pnpm --filter frontend run build
 
   code-checks:
     name: Code checks