fix(prefs): 解析失败时逐字段抢救+备份，避免升级改 schema 静默清空设置

[bigsongeth]

User description

问题

preferences.json 走严格反序列化：容器级 #[serde(default)] 能容忍「缺字段」（老文件读新版本 OK），但扛不住「字段仍在、值非法」——当某次版本更新改了 prefs 的 schema（如重命名枚举变体、改字段类型），老用户文件里的旧值在新版本里不再合法，整份 UserPreferences 解析失败 → 旧代码 unwrap_or_else(default) 静默回落默认值，用户随手改一项就把历史设置（热键、模型选择、风格…）永久覆盖成默认。

改动

read_preferences 严格解析失败时，不再静默重置，而是：

1. 原样备份坏文件为 preferences.corrupt-<ts>.json，永不销毁；
2. 逐字段抢救（UserPreferences::salvage_from_json_bytes）——把 JSON 当对象逐 key 试解析，只把值非法的字段回默认，其余仍合法的设置全部保留；
3. 写回抢救结果，得到干净可解析的文件。

新增测试 salvage_preserves_valid_fields_when_one_value_is_invalid：构造 defaultMode 非法但 dictationHotkey / activeAsrProvider 合法的文件，断言严格解析必失败、抢救后热键与 ASR 保住、仅非法字段回默认。

适用性

面向任何跨 schema 变更升级的用户，不局限于某种安装方式——只要某次发布改了 prefs 结构，此改动就能避免老用户设置被整份清空。维护者若认为发布流程已通过 serde default 覆盖此风险，可自行取舍。

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

---

PR Type

Bug fix, Enhancement

---

Description

• Backup unparseable preferences file before salvage

• Salvage valid fields by trying each JSON key individually

• Write back salvaged preferences to disk for clean future reads

• Add test verifying valid fields preserved when one field is invalid

---

Diagram Walkthrough

flowchart LR
  A["Start read_preferences"] --> B{"Parse strict?"}
  B -- "Success" --> C["Normal path"]
  B -- "Fail" --> D["Backup original to .corrupt file"]
  D --> E["Salvage: try each key"]
  E --> F["Write salvaged back"]
  F --> G["Return salvaged preferences"]

Loading

File Walkthrough

	Relevant files
Error handling	preferences.rs Handle parse failure with backup and salvage                          openless-all/app/src-tauri/src/persistence/preferences.rs Added backup of unparseable preferences file to .corrupt-.json Modified read_preferences to call salvage on parse failure instead of silently returning default Introduced backup_unparseable_preferences helper function +50/-2
Enhancement	types.rs Add per-field salvage logic to UserPreferences                      openless-all/app/src-tauri/src/types.rs Added salvage_from_json_bytes method to UserPreferences Implements per-field fallback: drops only invalid values while preserving valid ones Added test salvage_preserves_valid_fields_when_one_value_is_invalid +57/-0

---

[github-actions]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 3 🔵🔵🔵⚪⚪

🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected