Fix PII mapping add (plus button) not persisting and not dismissing notification

The plus button handler in the pre-send PII warning had two bugs:

1. It read/wrote mappings directly via the storage bridge (api.storage.local),
   bypassing the Storage module's encryption layer. When at-rest encryption was
   enabled, getStorageData returned an encrypted blob instead of an array,
   causing .push() to throw a TypeError that silently aborted the handler —
   the mapping was never saved and replaceInInput never ran.

2. Unlike the ignore button which immediately removes the DOM item, the plus
   button relied on a re-scan at 150ms to dismiss the notification. If
   replaceInInput didn't stick (e.g. React-controlled inputs), the re-scan
   found PII again and the notification persisted.

Fix: Route mapping creation through the background service worker via a new
'add:mapping' message handler (which uses Storage.addMapping with proper
encryption support), and immediately dismiss the notification item from the
DOM like the ignore button does.

https://claude.ai/code/session_01RfzvB5sHah326acr8Xa7Jn

src/background/service-worker.js

@@ -138,6 +138,12 @@ const messageHandlers = {
     sendResponse({ mappings });
   },
 
+  async 'add:mapping'(message, _sender, sendResponse) {
+    const newMapping = await Storage.addMapping(message.mapping);
+    const mappings = await Storage.getMappings();
+    sendResponse({ mapping: newMapping, mappings });
+  },
+
   async 'get:settings'(_message, _sender, sendResponse) {
     const settings = await Storage.getSettings();
     sendResponse({ settings });