[codex] Improve settings media background controls

[xiaokangzaina]

Summary

• Add chunked/stored media handling for custom backgrounds so large video backgrounds do not rely on oversized request bodies or IndexedDB.
• Extend the settings UI background controls with background type detection, custom background audio, media audio mode cycling, button sound feedback, startup/video loading animation, and card transparency persistence.
• Preserve background video playback progress across refreshes with local progress storage, guarded restore, heartbeat syncing, and backend progress-only preference merges.
• Add a 全关 media-audio mode (off) so video audio and background audio can both be disabled and persisted.

Why

The local plugin settings page needed to support common image/GIF/video/audio background workflows without failing on large uploads, losing playback position after refresh, or forcing one of the media audio sources to stay enabled.

Validation

ode --check pages/settings/app.js

• pytest tests/test_devkit_web.py -q -> 25 passed, 1 warning
• pytest -q -> 598 passed, 2 skipped, 2 failed, 1 warning

The full-suite failures are existing registry count assertions outside this PR's touched files:

• ests/test_registry.py::TestRegistryConsistency::test_tool_group_counts_match_readme: 文件系统 expected 12 tools, actual 13
• ests/test_registry.py::TestRegistryConsistency::test_total_tool_count: expected 64 tools, actual 65

[irmia2026]

感谢提交！功能层面实现得很用心，但想提一个方向性意见。

irmia_devkit_open 是一个开发工具箱，核心价值在命令行工具、代码分析、Git 管理能力。settings 页面是一个配置面板——用户配完参数就走，不需要在设置页停留。

目前这个 PR 5000+ 行主要投入在媒体背景、按钮音效、卡片透明度、启动动画等美化层面，偏重了。配置面板应保持简洁可用即可：

• 暗色模式切换 ✅（已有）
• 基础偏好持久化 ✅（已有）
• 自定义背景 → 建议精简为单张静态图或纯色，去掉视频/音频/GIF 的重型媒体逻辑
• 按钮音效、启动动画 → 建议移除，对配置面板没有实际价值

希望控制 scope，让 settings 页面回归「配置工具」的定位。感谢理解！