fix(ui): link #123 and @user refs to the repo's actual forge (#1596) (#1597)

* fix(ui): link #123 and @user refs to the repo's actual forge (#1596)

Bare issue and mention refs always linked to github.com, so GitLab and
GitHub Enterprise repos got links to the wrong forge. A new browser-safe
core helper, forgeRefLinks, picks the forge from the remote host:
GitHub-like hosts link to /issues/N, GitLab-like hosts to /-/issues/N,
unrecognized hosts render unlinked, and an absent host keeps the old
github.com behavior. The host classification is shared with the commit
avatar lookup. InlineMarkdown gains an optional repoHost prop threaded
from Viewer's repoInfo.host, and Pi's getRepoInfo now reports the host.

* fix(ui): keep github.com ref links for aliased and credentialed remotes

Using the remote host for the first time exposed remote spellings the
parser and classifier did not handle, so common github.com remotes
stopped linking to github.com: SSH-config aliases (github.com-work,
dotless work-gh), credentials in https URLs, ssh.github.com and
www.github.com, uppercase hosts, and bracketed hosts.

parseRemoteHost now strips URL userinfo, and core normalizes the host
before classifying it: lowercase, drop the port, fold the public
forges' alias hosts onto github.com / gitlab.com, and treat anything
that is not a plausible DNS name as absent, which keeps the legacy
github.com link. The commit-avatar lookup uses the same normalization.