feat: add default favicon resolution from public dir

[chenjiahan]

Summary

This PR adds default support for resolving public/favicon.[ext], a convention that helps simplify configuration for most web applications.

This behavior aligns with common defaults in tools like Create React App and other web toolings.

By default, .ico, .png, and .svg extensions are supported, which should cover the vast majority of use cases.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR adds automatic favicon resolution from the public directory, implementing a common convention that simplifies configuration for web applications. When no explicit favicon is configured, the system will look for favicon.ico, favicon.png, or favicon.svg files in public directories.

• Added default favicon resolution logic that searches public directories for favicon files
• Enhanced the HTML plugin to use the default favicon as a fallback when no explicit favicon is configured
• Added comprehensive test coverage for the new default favicon resolution functionality

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/core/src/plugins/html.ts	Implements default favicon resolution logic and integrates it as a fallback in the HTML plugin
e2e/cases/html/favicon/index.test.ts	Adds test cases for default favicon resolution and includes code formatting cleanup

Comments suppressed due to low confidence (1)

packages/core/src/plugins/html.ts:1

• Line 239 assigns defaultFavicon to itself instead of assigning faviconPath. This should be defaultFavicon = faviconPath;

import fs from 'node:fs';

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