Welcome to Flarum!
Regarding question 1, I don't think such extension exists at the moment. You could write it yourself (as it appears you tried), or find someone to create it for you.
Regarding question 2, Flarum code is pre-compiled, so it needs re-compiling after changes to the source files. Also any change you make in
vendor can be reverted anytime by Composer so we recommend not modifying anything there.
The recommended way to extend Flarum's frontend is through the extension API. There is an introduction here https://flarum.org/docs/extend/frontend.html but until we make the docs more complete, the best way to learn is to look at existing extensions.
If you must modify Flarum core files, you should setup a Composer workbench folder just like for extensions so your modified copy of Flarum core isn't overwritten by Composer. Then you can install NPM dependencies and run the compilation script just like for extensions to recompile Flarum core dist files.
Here's a list of resources to start with Flarum development https://discuss.flarum.org/d/1662-extension-developer-first-read