fix(reader): add test font, expand ranges, fix colors, add restore defaults

- Add Comic Sans MS as a test font option to verify font switching works
- Add !important to background-color and color in getCSS() to prevent
  book CSS from overriding user's reading theme colors
- Expand font size range from 12-24px to 10-40px, bump default to 18px
- Expand line height range from 1.0-2.5 to 0.8-3.0
- Add restoreDefaults() method that resets reading theme, font, size,
  line height, and justify/hyphenate to sensible defaults
- Add ↩️ restore defaults button in top bar underneath the settings gear

web/src/reader/settings-manager.ts

@@ -75,7 +75,7 @@ export function getDefaultSettings(): ReaderSettings {
     reading_font: "literata",
     tap_zone_size: 30,
     auto_scroll: false,
-    font_size: 16,
+    font_size: 18,
     line_height: 1.6,
     margin_width: 20,
     double_page_spread: false,