docs(env): add the .env.example the README already told people to copy

README.md line 72 says `cp .env.example .env`, but no such file existed and
.gitignore's `.env.*` rule would have swallowed it — hence the explicit
negation alongside it.

Documents the two variables the code actually reads, VITE_BASE_API_URL and
VITE_USE_MOCKS, rather than the longer list in the README, none of which is
referenced anywhere in apps/ or libs/. Notes that Vite resolves this from the
workspace root via envDir, and that Fayda needs nothing here beyond the callback
path matching the API's FAYDA_REDIRECT_URI.

.gitignore

@@ -13,6 +13,8 @@ coverage/
 # env
 .env
 .env.*
+# ...but the checked-in template must survive that rule.
+!.env.example
 
 # logs
 *.log