refactor: single source for writing styles + action labels; add theme module

- src/lib/actions.ts: canonical ACTIONS/ACTION_LABELS/WRITING_STYLES;
  context-menu styles derived (WRITING_STYLES minus Default). Replaces
  the three duplicated style arrays in popup, content, and background.
- src/lib/theme.ts: canonical palette. Existing hex literals in the
  entrypoints are swapped wholesale when light/dark theming lands, so
  the color plumbing is only rewritten once.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

entrypoints/content.ts

@@ -1,5 +1,6 @@
 import { defineContentScript } from 'wxt/utils/define-content-script';
 import { isExtensionValid, safeSendMessage } from '@lib/messaging';
+import { WRITING_STYLES } from '@lib/actions';
 
 export default defineContentScript({
   matches: ['<all_urls>'],
@@ -744,7 +745,7 @@ export default defineContentScript({
       modal.appendChild(body);
 
       // ─── Style selector row ───────────────────────────────────────────────
-      const STYLES = ['Default', 'Formal', 'Casual', 'Academic', 'Creative', 'Concise'];
+      const STYLES = WRITING_STYLES;
 
       const styleRow = document.createElement('div');
       Object.assign(styleRow.style, {