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/cards.js

@@ -19,14 +19,6 @@
     }
   }
 
-  function applyDarkMode(settings) {
-    if (window.RedditTweaks.isDarkMode(settings)) {
-      document.documentElement.classList.add("rt-dark");
-    } else {
-      document.documentElement.classList.remove("rt-dark");
-    }
-  }
-
   function applyBackground(settings) {
     if (settings.cardBackground === "shadcn") {
       document.documentElement.classList.add("rt-card-bg-shadcn");
@@ -68,7 +60,6 @@
 
   function apply(settings) {
     document.documentElement.classList.add("rt-card-enabled");
-    applyDarkMode(settings);
     applyBackground(settings);
     applyDensity(settings);
     applyGap(settings);
@@ -77,7 +68,6 @@
   }
 
   function updateSettings(settings) {
-    applyDarkMode(settings);
     applyBackground(settings);
     applyDensity(settings);
     applyGap(settings);