Is my AES-128-CTR implementation secure enough to encrypt private keys and mnemonic seed? I am building a cryptocurrency wallet online where everything is stored in the browser (No servers).

At this moment I am storing all private keys encrypted using AES-128-CTR I've chosen this method because is the standard for Ethereum keystore file format.

Is this algorithm secure to encrypt any other private key such Bitcoin (which is not hexadecimal)?

Would it be also secure to encrypt 12 words mnemonic or any other string?

Here is my implementation:
https://github.com/elevenyellow/coinfy/blob/master/miscellaneous/experiments/aes-ctr-128.js



Edit:
@SAI Peregrinus says on his response that my implementation is completely insecure which make me think, is the same for myetherwallet.com? Because my code is just a refactor of this:
https://github.com/kvhnuke/etherwallet/blob/44074daeef3f98ca15410158b5a5403ac734a08a/app/scripts/myetherwallet.js#L196



Edit2:
Studiying the geth (Ethereum go) client implementation I don't see any difference except for the parameter kdf.n where their value is 262144
and mine is 1024:

https://github.com/ethereum/go-ethereum/blob/bedf6f40aff0dd14bfab533a37c329ddc9a4bdd5/accounts/keystore/keystore_passphrase.go#L118
This is not secure at all. It's technically off-topic since it's a coding question, but this implementation contains many basic errors. One example is using an insecure random number generator, which utterly destroys the security of the encryption. Another is using AES-128-CTR without authentication.  Do not try to implement your own cryptographic primitives, it takes a ton of work and is prone to obscure errors causing total failure. Use an existing library that provides constructions based on secure primitives that do what you need. That pretty much means libsodium, libhydrogen, tweetnacl-js, or a similar library.

If you use libsodium there's a version compiled to JS: https://github.com/jedisct1/libsodium.js

Documentation here: https://download.libsodium.org/doc/

You're looking for the authenticated encryption functions (crypto_secretbox_easy and its helpers). 

Libsodium can encrypt whatever you want. Also the bitcoin private keys are just bitstrings, just like the Ethereum private keys. They can be represented in hexadecimal just as easily as any other encoding.