💽 fix: Exclude index page / from static cache settings (#7382)

* Disable default static caching for app's index page

* Update index.html related environment variables in `.env.example`

* Fix linting

* Update index.spec.js

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Co-authored-by: Danny Avila <danacordially@gmail.com>

.env.example

@@ -563,9 +563,9 @@ HELP_AND_FAQ_URL=https://librechat.ai
 #   users always get the latest version. Customize    #
 #   only if you understand caching implications.      #
 
-# INDEX_HTML_CACHE_CONTROL=no-cache, no-store, must-revalidate
-# INDEX_HTML_PRAGMA=no-cache
-# INDEX_HTML_EXPIRES=0
+# INDEX_CACHE_CONTROL=no-cache, no-store, must-revalidate
+# INDEX_PRAGMA=no-cache
+# INDEX_EXPIRES=0
 
 # no-cache: Forces validation with server before using cached version
 # no-store: Prevents storing the response entirely