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/gitea/auth.js

@@ -1,6 +1,7 @@
 import fs from 'fs';
 import path from 'path';
 import os from 'os';
+import { getProviderApiBaseUrl } from '../git-providers/config.js';
 
 const OPENCHAMBER_DATA_DIR = process.env.OPENCHAMBER_DATA_DIR
   ? path.resolve(process.env.OPENCHAMBER_DATA_DIR)
@@ -9,9 +10,16 @@ const OPENCHAMBER_DATA_DIR = process.env.OPENCHAMBER_DATA_DIR
 const STORAGE_DIR = OPENCHAMBER_DATA_DIR;
 const STORAGE_FILE = path.join(STORAGE_DIR, 'gitea-auth.json');
 
-// Gitea/Forgejo are self-hosted — there is deliberately NO default base URL.
-// The instance URL is always user-provided (see `normalizeBaseUrl`); auth.js
-// never invents a host for a stored account.
+// Gitea/Forgejo are self-hosted — there is deliberately NO built-in default
+// base URL. The instance URL is always user-provided (see `normalizeBaseUrl`);
+// auth.js never invents a host for a stored account. A configured
+// settings.json gitProviders.gitea.apiBaseUrl can act as the default for the
+// connect form, but stored accounts still require an explicit baseUrl.
+
+/** Effective default Gitea base URL: configured settings.json value, else null (no built-in default). */
+export function getGiteaDefaultBaseUrl() {
+  return getProviderApiBaseUrl('gitea');
+}
 
 function ensureStorageDir() {
   if (!fs.existsSync(STORAGE_DIR)) {