feat(ext/sw): get_field_history handler

Decrypts item and calls WASM get_field_history to extract tracked
field history for the popup's history view.

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

extension/src/service-worker/router/popup-only.ts

@@ -346,9 +346,13 @@ export async function handle(
       return { ok: true, data: result };
     }
 
-    // Handler for this case is added in Task 5.
-    case 'get_field_history':
-      return { ok: false, error: 'not_implemented' };
+    case 'get_field_history': {
+      const handle = session.getCurrent();
+      if (!handle || !state.gitHost) return { ok: false, error: 'vault_locked' };
+      const item = await vault.fetchAndDecryptItem(state.gitHost, handle, msg.id);
+      const history = state.wasm.get_field_history(JSON.stringify(item));
+      return { ok: true, data: { history } };
+    }
   }
 }