feat(web,ui): per-provider git forge API base URL and detection URLs

Configure a default API base URL per git provider (github/gitlab/gitea)
in settings.json gitProviders, with GitHub Enterprise support (Octokit
baseUrl + device-flow web origin derived from the API base), and replace
the client-side custom-domains list with server-persisted detection URL
chips (SSH/HTTPS forms normalized to hosts). The configured API base host
auto-counts as a detection host. Settings round-trip through the existing
/api/config/settings sanitizer; the UI store hydrates from server settings
with a one-time localStorage migration.

packages/web/server/lib/github/pr-status.js

@@ -2,6 +2,9 @@ import { stat } from 'node:fs/promises';
 import { getRemotes, getStatus } from '../git/index.js';
 import { resolveGitHubRepoFromDirectory } from './repo/index.js';
 import { noteIfGitHubRateLimit } from './rate-limit.js';
+import { getProviderApiBaseUrl, githubWebOriginFromApiBase } from '../git-providers/config.js';
+
+const getGitHubWebOrigin = () => githubWebOriginFromApiBase(getProviderApiBaseUrl('github'));
 
 const directoryExists = async (dir) => {
   if (!dir) return false;
@@ -295,7 +298,7 @@ const expandRepoNetwork = async (octokit, candidates) => {
       pushCandidate({
         owner: parent.owner.login,
         repo: parent.name,
-        url: parent.html_url || `https://github.com/${parent.owner.login}/${parent.name}`,
+        url: parent.html_url || `${getGitHubWebOrigin()}/${parent.owner.login}/${parent.name}`,
       }, candidate.remoteName, candidate.priority + 0.1);
     }
 
@@ -304,7 +307,7 @@ const expandRepoNetwork = async (octokit, candidates) => {
       pushCandidate({
         owner: source.owner.login,
         repo: source.name,
-        url: source.html_url || `https://github.com/${source.owner.login}/${source.name}`,
+        url: source.html_url || `${getGitHubWebOrigin()}/${source.owner.login}/${source.name}`,
       }, candidate.remoteName, candidate.priority + 0.2);
     }
   }