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>

content/tweaks/badges.js

@@ -6,12 +6,6 @@
   function applyColorsToContainer(container, settings, subredditName) {
     const prefix = window.RedditTweaks.getColorPrefix(settings);
 
-    if (window.RedditTweaks.isDarkMode(settings)) {
-      container.classList.add("rt-dark");
-    } else {
-      container.classList.remove("rt-dark");
-    }
-
     const entry = subredditName && settings.subredditColors[subredditName];
     const overrides = entry && entry[prefix];
 
@@ -98,10 +92,6 @@
       const subredditName = container.getAttribute("data-rt-subreddit");
       applyColorsToContainer(container, settings, subredditName);
     });
-
-    if (window.RedditTweaks.colorPopover && window.RedditTweaks.colorPopover.refreshTheme) {
-      window.RedditTweaks.colorPopover.refreshTheme(settings);
-    }
   }
 
   function apply(settings) {
@@ -113,8 +103,6 @@
   window.RedditTweaks.badgeTweak = {
     apply: apply,
     updateColors: updateColors,
-    isDarkMode: window.RedditTweaks.isDarkMode,
-    getColorPrefix: window.RedditTweaks.getColorPrefix,
     processNewPosts: processPosts,
   };
 })();