fix: improve password validation flow (#15132)

Refers to #14984 

Currently, password validation is done backend side and is not explicit
enough so it can be painful to create first users.
We'd like to make this validation easier - but also duplicate it
frontend side to make it smoother.

Flows involved : 
- First user set password
- New user set password
- Change password

---------

Co-authored-by: BrunoQuaresma <bruno_nonato_quaresma@hotmail.com>

coderd/coderd.go

@@ -1047,6 +1047,7 @@ func New(options *Options) *API {
 				r.Use(httpmw.RateLimit(options.LoginRateLimit, time.Minute))
 				r.Post("/login", api.postLogin)
 				r.Post("/otp/request", api.postRequestOneTimePasscode)
+				r.Post("/validate-password", api.validateUserPassword)
 				r.Post("/otp/change-password", api.postChangePasswordWithOneTimePasscode)
 				r.Route("/oauth2", func(r chi.Router) {
 					r.Route("/github", func(r chi.Router) {