feat: masked passwords UI with vault-gated reveal

Passwords imported from password managers are now:
- Displayed as dots (••••••••) by default in both options and popup
- Separated into their own "Passwords" section in options page
- Only revealable by entering the vault encryption password
- Always masked in the popup mappings list (no reveal there)

Options page:
- New Passwords section with locked/unlocked states
- Reveal button validates against vault encryption password
- Hide button re-masks all password values
- Password mappings excluded from the general Mappings table
- Delete and enable/disable controls work while masked

Popup:
- Password-category mappings show dots for real value
- Password category added to mapping add form dropdown

Storage:
- Added 'password' to default categories list

https://claude.ai/code/session_01SWSwDfMVij53bCTNSCLMwn

src/popup/popup.html

@@ -133,6 +133,7 @@
             <option value="ssn">SSN</option>
             <option value="dob">DOB</option>
             <option value="domain">Domain</option>
+            <option value="password">Password</option>
             <option value="general">General</option>
           </select>
           <label class="checkbox-label">