Replace token-paste with native device-code/PKCE provider sign-in

Implement the official OAuth login flows natively (ported from pi-ai's
reference): GitHub Copilot device-code, GPT account (ChatGPT Codex) and
Google Antigravity authorization-code + PKCE via a loopback callback server.
Obtained credentials are stored as JSON through the existing secret vault.
Onboarding and Settings now run the real sign-in instead of pasting an
access token.

AgentCompany-New/src/shared/ipc.ts

@@ -21,6 +21,8 @@ export const IPC_CHANNELS = {
   saveProviderCredential: "agentcompany:save-provider-credential",
   revokeProviderCredential: "agentcompany:revoke-provider-credential",
   launchProviderSignIn: "agentcompany:launch-provider-sign-in",
+  startProviderOAuth: "agentcompany:start-provider-oauth",
+  cancelProviderOAuth: "agentcompany:cancel-provider-oauth",
   checkProviderHealth: "agentcompany:check-provider-health",
   updateProviderPolicy: "agentcompany:update-provider-policy",
   openExternalUrl: "agentcompany:open-external-url",