feat: add 'domain' category to mappings dropdown

Users can now label mappings as 'domain' (e.g. mycompany.com →
example.com). This is a category label for organization — the
substitution works the same as any other mapping.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/lib/storage.js

@@ -21,7 +21,7 @@ const DEFAULT_SETTINGS = {
   secretScanning: true,
   maxLogEntries: 200,
   customDomains: [],
-  categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'general'],
+  categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'domain', 'general'],
 };
 
 const Storage = {

src/options/options.html

@@ -102,6 +102,7 @@
           <option value="address">Address</option>
           <option value="ssn">SSN</option>
           <option value="dob">DOB</option>
+          <option value="domain">Domain</option>
           <option value="general">General</option>
         </select>
         <label class="checkbox-label">

src/popup/popup.html

@@ -120,6 +120,7 @@
             <option value="address">Address</option>
             <option value="ssn">SSN</option>
             <option value="dob">DOB</option>
+            <option value="domain">Domain</option>
             <option value="general">General</option>
           </select>
           <label class="checkbox-label">