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/opencode/settings-helpers.js

@@ -1,3 +1,5 @@
+import { sanitizeGitProviders } from '../git-providers/config.js';
+
 export const createSettingsHelpers = (dependencies) => {
   const {
     normalizePathForPersistence,
@@ -481,6 +483,10 @@ export const createSettingsHelpers = (dependencies) => {
       const trimmed = candidate.gitModelId.trim();
       result.gitModelId = trimmed.length > 0 ? trimmed : undefined;
     }
+    const gitProviders = sanitizeGitProviders(candidate.gitProviders);
+    if (gitProviders) {
+      result.gitProviders = gitProviders;
+    }
     if (typeof candidate.pwaAppName === 'string') {
       result.pwaAppName = normalizePwaAppName(candidate.pwaAppName, undefined);
     }