refactor: decompose CSS, unify dark mode, extract shared modules

Structural cleanup based on thermo-nuclear code quality review:
- Extract shared color-picker.css and color-popover.css from badges.css (611→85 lines)
- Extract shared lib/presets.js for preset CRUD used by popup and popover
- Unify dark mode to single applyDarkModeClass on <html>, removing 4 scattered functions
- Fix stale closure in startPostObserver, data-driven reload key check
- Remove dead exports, getHelpers wrapper, duplicated colorToHex
- Replace DOM expando properties with state array in popup.js

Net: -318 lines of duplication, no file over 401 lines.
Co-authored-by: Cursor <cursoragent@cursor.com>

lib/settings.js

@@ -76,6 +76,14 @@
     return isDarkMode(settings) ? "dark" : "light";
   }
 
+  function applyDarkModeClass(settings) {
+    if (isDarkMode(settings)) {
+      document.documentElement.classList.add("rt-dark");
+    } else {
+      document.documentElement.classList.remove("rt-dark");
+    }
+  }
+
   window.RedditTweaks = window.RedditTweaks || {};
   window.RedditTweaks.DEFAULTS = DEFAULTS;
   window.RedditTweaks.loadSettings = loadSettings;
@@ -83,4 +91,5 @@
   window.RedditTweaks.resetSettings = resetSettings;
   window.RedditTweaks.isDarkMode = isDarkMode;
   window.RedditTweaks.getColorPrefix = getColorPrefix;
+  window.RedditTweaks.applyDarkModeClass = applyDarkModeClass;
 })();