feat(reader): add light/dark mode toggle for reading themes

Add explicit light/dark mode toggle switch to the reading theme settings.
The reading mode defaults based on the chrome theme (dark chrome themes
like tokyo-night default to dark reading mode).

- Add readingMode property to readerShell Alpine component
- Add toggleReadingMode() method that toggles dark class on body
- Add detectChromeDarkMode() to infer default from chrome theme
- Update applyTheme() to add/remove dark class and persist reading_mode
- Add toggle switch UI in settings panel (blue pill style, next to
  Reading Theme heading)
- Add reading_mode to default settings in settings-manager

web/src/reader/settings-manager.ts

@@ -70,7 +70,8 @@ export function getDefaultSettings(): ReaderSettings {
     chrome_behavior: "auto-hide",
     progress_mode: "pages",
     chrome_theme: "tokyo-night",
-    reading_theme: "dark",
+    reading_theme: "light",
+    reading_mode: "dark",
     reading_font: "literata",
     tap_zone_size: 30,
     auto_scroll: false,