fix: prevent false 'model error' popup during startup

[wei-yao]

Problem

When the app starts, a false "❌ 模型错误" popup appears when the user clicks the mic button, even though:

• FunASR models are downloaded and working
• AI optimization is disabled in settings

Root Cause

There is a startup race condition. initializeAtStartup() runs two async Python checks (findPythonExecutable, checkFunASRInstallation) before calling preInitializeModels() which sets initializationPromise. During this brief window:

1. checkStatus() returns initializing: false (because initializationPromise is still null)
2. useModelStatus.js sees: models downloaded + server not ready + initializing: false
3. Falls into the else branch → sets stage: 'error'
4. User clicks mic → toggleRecording() sees stage === 'error' → shows popup

Fix

funasrManager.js — also report initializing: true before startup has completed:

// before
initializing: this.initializationPromise !== null

// after
initializing: this.initializationPromise !== null || !this.isInitialized

useModelStatus.js — when models are downloaded but server isn't ready yet, treat as 'loading' not 'error'. Reserve 'error' for genuine failures (FunASR not installed, models missing):

// before
} else if (serverStatus.initializing) {
  stage: 'loading'
} else {
  stage: 'error'   // ← triggered too eagerly
}

// after
} else if (serverStatus.initializing || serverStatus.models_downloaded) {
  stage: 'loading' // ← models downloaded = still starting up
} else {
  stage: 'error'   // ← only when truly broken
}