Fix error serialization in frontend notifications

[htrex]

Summary

Fixes frontend displaying [object Object] instead of actual error messages when the backend returns validation errors.

Problem

When the backend returns a Pydantic validation error (HTTP 422), the detail field is an array of objects, not a string:

{
  "detail": [
    {
      "type": "less_than_equal",
      "loc": ["body", "chunk_size"],
      "msg": "Input should be less than or equal to 500",
      "input": 510
    }
  ]
}

The frontend passed errorResult.detail directly to new Error(), which called .toString() on the array and displayed "[object Object]" to the user.

Steps to reproduce

1. Enable text splitting in the UI
2. Set the chunk size slider to a value above 500 (e.g. 510)
3. Click "Generate Speech"
4. Observe [object Object] in the error notification

Solution

Added a formatErrorDetail() utility function that handles all API error response formats:

• String (standard HTTPException): returned as-is
• Array (Pydantic 422 validation errors): extracts the .msg field from each entry and joins them with "; "
• Object (unexpected format): falls back to JSON.stringify()

Applied to all 4 error handling paths in script.js:

• UI state save errors
• Model configuration errors
• TTS generation errors
• Settings reset errors

After fix

The error notification now correctly displays: "Input should be less than or equal to 500"

Test Plan

• Set chunk size > 500, generate speech → verify readable error message appears
• Trigger a standard server error (e.g. no voice selected) → verify error still displays correctly
• Save settings with invalid values → verify error message is readable