fix: 配置模块可靠性加固 (#88)

* fix: harden config module against corruption, I/O failures, and silent data loss

Risks discovered in security review and their mitigations:

1. [HIGH] Invalid enum deserialization from manual JSON edits
   - Add AppConfig.Sanitize() + AuthCredential.Sanitize() to repair invalid
     AuthScheme values and null nested objects on load

2. [HIGH] Narrow exception catch in Load() — only caught JsonException
   - Expanded to catch IOException + UnauthorizedAccessException to handle
     disk-full and permission-denied scenarios gracefully

3. [HIGH] Fire-and-forget SaveAsync exceptions silently lost
   - Added ConfigService.SafeFireAndForgetSave() that logs failures to Trace
   - Updated all 12 fire-and-forget call sites in App + 4 ViewModels

4. [MEDIUM] Unnecessary disk write on every startup
   - OnAutoUploadEnabledChanged in PatchViewModel constructor triggered
     SaveAsync during init. Added _initialized guard to skip during construction

5. [LOW] Corrupted config silently overwrites backup before recovery attempt
   - Sanitize() is called on recovered configs before re-saving to disk

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

* fix: use synchronous save on window close to prevent state loss

Window close handler used SafeFireAndForgetSave (Task.Run), which creates
a race: the process may exit before the thread-pool write completes,
silently losing window size/position/maximized state on next launch.

Changed to synchronous ConfigService.Save() which completes in <1ms for
the tiny (<2KB) config file. Also promoted Save() from private to public
and added it to IConfigService for this exact shutdown scenario.

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

* chore: final code review cleanup before release

- Add missing Sanitize() calls in LoadAsync() backup-recovery and normal paths
- Remove unused System.Threading.Tasks import from App.axaml.cs
- Remove unused BuildUrl dead code from HttpUploadService
- Remove unnecessary 'new' keyword on AppLanguageManager.Instance
- Fix LoadAsync() to catch IOException/UnauthorizedAccessException

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

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: JusterZhu