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/ui/src/lib/settings/search.ts

@@ -611,6 +611,13 @@ const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     titleKey: 'settings.openchamber.worktrees.setup.waitForCommands',
     keywords: ['worktree', 'setup commands', 'bootstrap', 'wait'],
   },
+  {
+    id: 'projects.git-providers',
+    page: 'projects',
+    titleKey: 'settings.projects.page.gitProviders.title',
+    descriptionKey: 'settings.projects.page.gitProviders.description',
+    keywords: ['github', 'gitlab', 'gitea', 'api base url', 'self-hosted', 'override', 'enterprise', 'server'],
+  },
   {
     id: 'remote-instances.client-auth',
     page: 'remote-instances',