feat: multi-account support and OAuth tokens management

[benzntech]

Summary

This PR adds multi-account load balancing support and dedicated OAuth token management to the admin UI.

Changes

Commit 1: Multi-account load balancing with UI management

• Add multi-account support to provider configuration with round-robin balancing
• Implement account management UI in admin panel (add/edit/remove accounts)
• Add UUID and atomic write dependencies for account ID generation and persistence
• Pre-fill auth_type when editing providers to fix OAuth display bug
• Show account count on provider cards in admin panel
• Update README with multi-account features documentation

Commit 2: Dedicated OAuth Tokens management tab

• Create new 'OAuth Tokens' tab in admin UI navigation
• Move OAuth token management to dedicated page for better organization
• Keep OAuth tokens section in Settings tab as well (available in both places)
• Update loadOAuthTokens() to populate both Settings and OAuth Tokens tabs
• Add null check in handleRoute() to prevent errors when tab content doesn't exist
• Both tabs show same token list with refresh and delete buttons

Files Modified

• Cargo.toml - Added UUID and atomic_write dependencies
• README.md - Updated documentation with multi-account features
• src/cli/mod.rs - CLI support for multi-account management
• src/providers/mod.rs - Provider management with multi-account logic
• src/providers/registry.rs - Registry updates for account handling
• src/server/admin.html - UI enhancements for accounts and OAuth tokens

Architecture

All changes follow the existing localStorage-based state management pattern:

• Page load: Fetch from server → save to localStorage
• All operations: Update localStorage only
• Save buttons: Sync localStorage → server