feat(xai): add Grok OAuth device-flow backend and proxy routing

Add an xAI OAuth manager using the OAuth 2.0 Device Authorization Grant
with endpoints resolved from xAI's OIDC discovery document. All HTTP goes
through the app-managed proxy client.

- Managed provider kind xai_oauth: forced openai_responses wire format,
  pinned api.x.ai base URL, bearer injection gated to the xAI origin,
  tokens registered for log redaction, single-auth-key takeover policy.
- Token cache cannot bypass account state: cache hits re-validate account
  usability, refresh commits run under the mutation lock with a
  refresh-token CAS check, and pending logins are re-checked before an
  account is persisted.
- Refresh classification: 401/403 with any body and 400 with a non-JSON
  body mark the account for re-auth; 429/5xx stay transient.
- Shared auth_* commands dispatch to xAI with guard types mirroring the
  Copilot/Codex branches.

src-tauri/src/provider.rs

@@ -71,6 +71,10 @@ impl Provider {
         self.provider_type() == Some("codex_oauth")
     }
 
+    pub fn is_xai_oauth(&self) -> bool {
+        self.provider_type() == Some("xai_oauth")
+    }
+
     pub fn is_github_copilot(&self) -> bool {
         self.provider_type() == Some("github_copilot")
             || self.claude_base_url_contains("githubcopilot.com")
@@ -79,6 +83,7 @@ impl Provider {
     pub fn uses_managed_account_auth(&self) -> bool {
         self.is_github_copilot()
             || self.is_codex_oauth()
+            || self.is_xai_oauth()
             || self.claude_base_url_contains("chatgpt.com/backend-api/codex")
     }