fix: webapp - wsl shell source not found

A bug was found (see GitHub Issue srbhr#153), where when a user attempting to run the `npm run dev` or `npm run fastapi-dev` commands from within a (Windows) WSL Ubunutu Shell window, encounters the following error, which prevents the backend FastAPI server from running:

`sh: 1: source: not found`

Given it appears that `source` is a command built-in and runnable on Bash and ZSH terminal environments, it is not available to use within the WSL Ubuntu shell by default.

To fix this, a more universal substitute can be used in place of `source`, which is the period `.`, which performs the same function in Bash, ZSH, and WSL Ubunutu terminal / shell environments.

webapp/package.json

@@ -4,7 +4,7 @@
   "private": true,
   "scripts": {
     "next-dev": "next dev",
-    "fastapi-dev": "source ../env/bin/activate && pip install -r ../requirements.txt && python -m uvicorn backend.api.index:app --reload",
+    "fastapi-dev": ". ../env/bin/activate && pip install -r ../requirements.txt && python -m uvicorn backend.api.index:app --reload",
     "dev": "concurrently \"npm run next-dev\" \"npm run fastapi-dev\"",
     "build": "next build",
     "start": "next start",