fix(reader): dark text on dark themes and add toggle icons

Two fixes:

1. Remove base typography block from foliate-themes.css. The html/body
   rules were unlayered CSS that overrode the chrome theme's layered
   body styles, causing dark reading theme text colors (--reader-text)
   to apply to the outer chrome UI on dark backgrounds. These styles
   are only meant for the shadow DOM, which getCSS() already handles.

2. Move missing typography rules (img, blockquote, a, p orphans/widows)
   into getCSS() so the shadow DOM still gets them.

3. Add sun/moon emoji indicators to the light/dark toggle switch.

web/static/foliate-themes.css

@@ -256,46 +256,4 @@
   --reader-link: #268bd2;
   --reader-selection: #073642;
 }
-/* ==========================================
-   BASE TYPOGRAPHY (applies to all themes)
-   ========================================== */
-html,
-body {
-  background-color: var(--reader-bg);
-  color: var(--reader-text);
-  font-family: var(--reader-font, "Literata", serif);
-  font-size: var(--reader-font-size, 16px);
-  line-height: var(--reader-line-height, 1.6);
-  margin: var(--reader-margin, 20px);
-  padding: var(--reader-margin, 20px);
-}
-/* Links */
-a {
-  color: var(--reader-link);
-  text-decoration: underline;
-}
-/* Text selection */
-::selection {
-  background-color: var(--reader-selection);
-}
-/* Images */
-img,
-svg,
-video {
-  max-width: 100%;
-  height: auto;
-  display: block;
-  margin: 0.5em auto;
-}
-/* Prevent orphans/widows */
-p {
-  orphans: 3;
-  widows: 3;
-}
-/* Block quotes */
-blockquote {
-  margin: 1em 0;
-  padding-left: 1em;
-  border-left: 3px solid var(--reader-link);
-  font-style: italic;
-}
+