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>

popup/popup.html

@@ -3,6 +3,7 @@
 <head>
   <meta charset="UTF-8">
   <link rel="stylesheet" href="popup.css">
+  <link rel="stylesheet" href="../content/styles/color-picker.css">
 </head>
 <body>
   <div class="popup-container">
@@ -118,6 +119,7 @@
     </div>
   </div>
   <script src="../lib/settings.js"></script>
+  <script src="../lib/presets.js"></script>
   <script src="../content/tweaks/color-picker.js"></script>
   <script src="popup.js"></script>
 </body>