[Refactor/Chore] Optimize the oversized in-site notification background asset

从 web/public/in-site-message/header-bg.svg 和 web/app/components/app/in-site-message/index.tsx 开始；检查 fallback URL 和自定义 title_pic_url 的使用方式，然后测量当前 asset 和 production transfer。优化 fallback，同时检查普通和高 DPI 下的显示效果，并保留自定义图片的行为。完成标准是 layout 未发生变化，并记录大小和 transfer 的优化前后测量结果。

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Description

Optimize web/public/in-site-message/header-bg.svg, the fallback background for the small in-site notification header.

The SVG declares a 360×128 viewport but embeds a 4096×1625 PNG of 3,577,618 bytes as base64. The SVG is 4,772,282 bytes; estimated gzip remains 3,579,965 bytes. The public file is copied unchanged to the client build.

web/app/components/app/in-site-message/index.tsx uses this URL as DEFAULT_HEADER_BG_URL. The notification wrapper is Cloud-edition gated, and the fallback applies when title_pic_url is undefined. This is not a claim that every page or Community deployment downloads the image; the local Community page did not exercise a real notification download.

Reproduce by checking the SVG size, decoding its embedded PNG dimensions, and displaying a Cloud notification without a custom header image.

Motivation

A decorative header should not require a multi-megabyte raster. Replace the embedded image with an appropriately sized optimized asset, or an equivalent lightweight design, while preserving its appearance.

Acceptance criteria:

• Compare the background at normal and high-DPI display scales.
• Preserve custom header-image behavior and the fallback layout.
• Record before/after asset size and actual production compressed transfer.

Additional Context

Observed on a local Dify 1.17.1 development branch based on 06e473015a9a1abb71da88ac0a0e92d96a992008, with local DevTools integration patches; Vinext 1.0.0-beta.10, Vite+ core 0.3.2, Node 24.21.0. Recheck against current main before implementation. Build measurements are diagnostic baselines, not controlled production network benchmarks.