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/lib/storage.js

@@ -40,7 +40,7 @@ const DEFAULT_SETTINGS = {
   autoAddDetected: true,
   maxLogEntries: 200,
   customDomains: [],
-  categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'domain', 'general'],
+  categories: ['name', 'email', 'phone', 'address', 'ssn', 'dob', 'domain', 'password', 'general'],
   browserSync: false,
 };