fix(appearance): 完善深色模式和阅读页面对比度

- 区分按钮填充与图标颜色，统一书架、书源、搜索和阅读设置的选中态
- 调整编辑器语法高亮、键盘工具栏及统计卡片的浅深色显示
- 适配 RSS 正文和登录网页背景，保留书源自定义样式和页面状态
- 补充外观切换、页面对比度和管理页面的回归测试

App/Features/Sources/SourceLoginView.swift

@@ -1,5 +1,6 @@
 @preconcurrency import WebKit
 import SwiftUI
+import UIKit
 
 struct SourceLoginView: View {
     let source: BookSource
@@ -74,6 +75,9 @@ private struct SourceLoginWebView: UIViewRepresentable {
         let configuration = WKWebViewConfiguration()
         configuration.websiteDataStore = .default()
         let webView = WKWebView(frame: .zero, configuration: configuration)
+        webView.isOpaque = false
+        webView.backgroundColor = UIColor(ReadingTheme.canvas)
+        webView.scrollView.backgroundColor = UIColor(ReadingTheme.canvas)
         webView.navigationDelegate = context.coordinator
         context.coordinator.installExistingCookies(in: webView) { webView.load(request) }
         return webView