feat(web,ui): per-project git provider API base URL overrides

Per provider (github|gitlab|gitea) a project override stored in
~/.config/openchamber/projects/<projectId>.json under gitProviders wins
over the global settings.json value (precedence: project override > global
> built-in default). Server forge routes resolve the override per request
directory (worktree-aware via git-common-dir + containment + path fallback,
60s TTL cache); the override host is also accepted for remote parsing and
client detection. New GET/PUT /api/projects/:projectId/git-providers route;
client openchamberConfig preserves the server-owned gitProviders key;
Projects page gains a Git provider API base URLs section; detection store
hydrates per-project overrides (memory-only, server-authoritative).

packages/web/server/lib/walkthrough/pull-request.js

@@ -15,7 +15,7 @@ const GITLAB_DIFFS_MAX_PAGES = 10;
  * base branch is not part of it.
  */
 async function getGitHubPullRequestDiff(directory, number) {
-  const octokit = getOctokitOrNull();
+  const octokit = getOctokitOrNull(directory);
   if (!octokit) {
     throw Object.assign(new Error('Connect a GitHub account to review pull requests'), {
       statusCode: 401,