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/content/injector.js

@@ -193,6 +193,26 @@
       if (event.data?.type === 'ss:storage-set') {
         await api.storage.local.set({ [event.data.key]: event.data.value });
       }
+
+      if (event.data?.type === 'ss:add-mapping') {
+        try {
+          const response = await api.runtime.sendMessage({
+            type: 'add:mapping',
+            mapping: event.data.mapping,
+          });
+          window.postMessage({
+            type: 'ss:add-mapping-result',
+            id: event.data.id,
+            mappings: response?.mappings || [],
+          }, '*');
+        } catch {
+          window.postMessage({
+            type: 'ss:add-mapping-result',
+            id: event.data.id,
+            mappings: [],
+          }, '*');
+        }
+      }
     });
   }