refactor: declarative context-menu registry; named selection threshold

CONTEXT_MENU_ENTRIES maps every menu id to {action, style, parentId,
title}, replacing the split('-') id parsing that only worked because no
name contained a hyphen. Foreign menu ids are now ignored explicitly.
Parity pinned by tests enumerating all 30 previous ids. The 10-char
toolbar trigger threshold is now the named MIN_SELECTION_LENGTH.

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

entrypoints/background.ts

@@ -1,6 +1,6 @@
 import { defineBackground } from 'wxt/utils/define-background';
 import type { AnalyzePayload, LexAIConfig, LexAIResponse } from '@lib/types';
-import { ACTIONS, ACTION_LABELS, CONTEXT_MENU_STYLES } from '@lib/actions';
+import { CONTEXT_MENU_ENTRIES, findContextMenuEntry } from '@lib/actions';
 import { decryptApiKey } from '@lib/crypto';
 import { callProvider, getSystemPrompt, listModels } from '@lib/providers';
 
@@ -40,35 +40,26 @@ export default defineBackground(() => {
   // ─── Context menus ───────────────────────────────────────────────────────
   chrome.runtime.onInstalled.addListener(() => {
     chrome.contextMenus.removeAll(() => {
-      ACTIONS.forEach(action => {
+      CONTEXT_MENU_ENTRIES.forEach((entry) => {
         chrome.contextMenus.create({
-          id: `lexai-${action}`,
-          title: `⚡ LexAI: ${ACTION_LABELS[action]}`,
+          id: entry.id,
+          parentId: entry.parentId,
+          title: entry.title,
           contexts: ['selection'],
         });
-        CONTEXT_MENU_STYLES.forEach(style => {
-          chrome.contextMenus.create({
-            id: `lexai-${action}-${style.toLowerCase()}`,
-            parentId: `lexai-${action}`,
-            title: style,
-            contexts: ['selection'],
-          });
-        });
       });
     });
   });
 
   chrome.contextMenus.onClicked.addListener((info, tab) => {
     if (!info.selectionText || !tab?.id) return;
-    // Parse action and style from menuItemId e.g. "lexai-fix-formal"
-    const parts = info.menuItemId.toString().replace('lexai-', '').split('-');
-    const action = parts[0];
-    const style = parts[1] ? parts[1].charAt(0).toUpperCase() + parts[1].slice(1) : 'Default';
+    const entry = findContextMenuEntry(info.menuItemId.toString());
+    if (!entry) return;
     chrome.tabs.sendMessage(tab.id, {
       type: 'lexai-context-menu',
-      action,
+      action: entry.action,
       text: info.selectionText,
-      style,
+      style: entry.style,
     });
   });