feat: add automatic account switching when usage threshold reached

[122yjs]

Summary

This PR adds an automatic account switching feature that automatically rotates to a different account when the current account's usage reaches a configurable threshold.

Features

Auto-Switch Monitor

• Background monitor that periodically checks account usage
• Automatically switches to the next available account when threshold is exceeded (default 95%)
• Skips switching while Codex is running to avoid interrupting active sessions
• Respects weekly (secondary) limits in addition to 5-hour (primary) limits

Configuration

• Threshold percentage (default 95%)
• Check interval in seconds (default 60s)
• Option to respect weekly limits
• Priority account ordering
• Account exclusion list

UI

• New "Auto-Switch" button in header
• Settings modal for configuration
• Status indicator (running/stopped)
• Switch history display

Implementation

Backend (Rust)

• src-tauri/src/auto_switch.rs - Background monitoring logic
• src-tauri/src/commands/auto_switch.rs - Tauri commands
• AutoSwitchConfig and AutoSwitchEvent types in types.rs
• Config stored in ~/.codex-switcher/auto_switch_config.json

Frontend (TypeScript/React)

• src/hooks/useAutoSwitch.ts - React hook for state management
• src/components/AutoSwitchSettings.tsx - Settings modal component

How It Works

┌─────────────────────────────────────┐
│     Auto-Switch Monitor            │
│  (checks every N seconds)          │
└──────────────┬──────────────────────┘                 ▼
┌─────────────────────────────────────┐
│ 1. Check if enabled & Codex not running  │
│ 2. Get current account usage             │
│ 3. If usage >= threshold:                │
│    - Find best available account          │
│    - Switch via ~/.codex/auth.json        │
│    - Log switch event                     │
└─────────────────────────────────────┘

Testing

Tested locally with multiple accounts. The monitor correctly:

• Detects when Codex is running and skips checks
• Switches accounts when threshold is reached
• Respects priority order and exclusions
• Handles API errors gracefully