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/background.ts

@@ -1,6 +1,7 @@
 import { defineBackground } from 'wxt/utils/define-background';
 import nacl from 'tweetnacl';
 import type { AnalyzePayload, LexAIConfig, LexAIResponse } from '@lib/types';
+import { ACTIONS, ACTION_LABELS, CONTEXT_MENU_STYLES } from '@lib/actions';
 
 // ─── Fetch with timeout ───────────────────────────────────────────────────────
 
@@ -431,25 +432,15 @@ export default defineBackground(() => {
   console.log('LexAI background service worker started');
 
   // ─── Context menus ───────────────────────────────────────────────────────
-  const CONTEXT_ACTIONS = ['fix', 'rephrase', 'shorten', 'expand', 'explain'] as const;
-  const CONTEXT_ACTION_LABELS: Record<string, string> = {
-    fix: 'Fix Grammar',
-    rephrase: 'Rephrase',
-    shorten: 'Shorten',
-    expand: 'Expand',
-    explain: 'Explain',
-  };
-  const CONTEXT_STYLES = ['Formal', 'Casual', 'Academic', 'Creative', 'Concise'];
-
   chrome.runtime.onInstalled.addListener(() => {
     chrome.contextMenus.removeAll(() => {
-      CONTEXT_ACTIONS.forEach(action => {
+      ACTIONS.forEach(action => {
         chrome.contextMenus.create({
           id: `lexai-${action}`,
-          title: `⚡ LexAI: ${CONTEXT_ACTION_LABELS[action]}`,
+          title: `⚡ LexAI: ${ACTION_LABELS[action]}`,
           contexts: ['selection'],
         });
-        CONTEXT_STYLES.forEach(style => {
+        CONTEXT_MENU_STYLES.forEach(style => {
           chrome.contextMenus.create({
             id: `lexai-${action}-${style.toLowerCase()}`,
             parentId: `lexai-${action}`,