feat: polish settings custom background UI

[xiaokangzaina]

Summary

• Add custom background upload/preset switching support in the settings UI.
• Convert per-tool toggles into click-to-toggle tool cards while keeping group-level switches.
• Clean up and consolidate custom-background glass styling into one maintainable CSS layer.
• Make config rows, contact rows, stat cards, and tool cards consistently ultra-transparent with theme-colored active borders.

Checks

• syntax_check pages/settings/app.js passed.
• JS linter unavailable in the local environment, so lint was skipped.

[irmia2026]

Thanks for the PR! The canvas blur-crop approach is clever, and the performance optimizations (will-change/contain) are thoughtful. A few issues to address:

🔴 Important

1. Toast 与实际行为矛盾

handleCustomBackgroundUpload 第 318 行 toast："深色/浅色/自动模式仍可切换"。但 cycleAppearanceMode 和 applyAppearance 都强制锁定 dark 模式，实际无法切换。建议改为：

showToast("自定义背景已启用（深色模式）");

2. switchToPresetBackground 丢失用户的 appearance 偏好

自定义背景启用时 appearanceMode 被强制改为 "dark"。切回预设后 applyAppearance(appearanceMode) 永远传 "dark"，用户之前的 "light" 或 "auto" 设置丢失。建议在启用自定义背景前保存原 appearanceMode（如 previousAppearanceMode），切回时恢复。

🟡 Minor

3. prefers-reduced-motion 太激进

当前 * { animation: none !important; transition: none !important; } 影响页面所有元素。建议限定为自定义背景模式下的卡片元素，避免波及其他 UI 组件。

4. refreshThemeControls 被三重调用

applyCustomBackground → applyPalette (内部调 refreshThemeControls) → applyAppearance (内部也调) → 末尾再手动调一次。功能正确但可精简。

✅ 做得好的

• Canvas 双层模糊裁切 + shadow 描边方案很漂亮
• 12MB 限制 + CUSTOM_BACKGROUND_SIZE 常量化
• aria-pressed + focus-visible 无障碍处理
• will-change/contain/translateZ 性能优化到位
• button:disabled 样式完善

[irmia2026]

@xiaokangzaina Thanks for the follow-up fix! Re-reviewing 4e68729:

#1 (Toast): Button now properly disabled with title "自定义背景下已锁定深色模式" and label showing "锁定". This correctly communicates that custom backgrounds intentionally lock dark mode — not a bug but a design decision. Toast copy could be updated to match, minor and not a blocker.

#2 (previousAppearanceMode): Now persisted both front-end and back-end. Looks correct.

#3 (reduced-motion): Still suggest scoping to custom-bg cards only — not blocking.

#4 (refreshThemeControls triple call): Now single-called from applyPalette. Clean.

No blockers. Approved.