fix(ssl): support IPv6 hosts in panel SSL self-signed certificate flow (#12652)

Enabling Panel SSL with the self-sign provider rejected IPv6 hosts with
"domain format invalid". Two coupled bugs caused this:

1. The frontend extracted the host from window.location.href with
   href.split('//')[1].split(':')[0]. For an IPv6 URL like
   https://[::1]:1234 that yields '[' \u2014 not a valid host \u2014 because
   the second split splits on the first colon inside the bracketed
   address. Use window.location.hostname, which natively returns the
   bracket-stripped IPv6 host.

2. The backend ObtainSSL flow used net.ParseIP(domain) directly. Even if
   the frontend sent the bracketed form ('[::1]'), net.ParseIP rejects
   brackets, so the value flowed into IsValidDomain() and failed the
   regex.

Add common.ParseIPLoose() that accepts both bare and bracketed IPv6 in
addition to bare IPv4. Use it at both call sites in ObtainSSL (renew
path and create path). A unit test guards the regression.

Files:
  - agent/utils/common/common.go               (new ParseIPLoose helper)
  - agent/utils/common/parse_ip_test.go        (12 cases, all green)
  - agent/app/service/website_ca.go            (call sites switched)
  - frontend/src/views/setting/safe/ssl/index.vue
                                                (host extraction fix)

Fixes #12646

Signed-off-by: Sanjay Santhanam <51058514+Sanjays2402@users.noreply.github.com>