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>

manifest.json

@@ -16,9 +16,10 @@
   "content_scripts": [
     {
       "matches": ["*://old.reddit.com/*", "*://www.reddit.com/*"],
-      "css": ["content/styles/badges.css", "content/styles/cards.css", "content/styles/fonts.css"],
+      "css": ["content/styles/badges.css", "content/styles/color-popover.css", "content/styles/color-picker.css", "content/styles/cards.css", "content/styles/fonts.css"],
       "js": [
         "lib/settings.js",
+        "lib/presets.js",
         "content/tweaks/badges.js",
         "content/tweaks/color-picker.js",
         "content/tweaks/color-popover.js",