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.js

@@ -585,7 +585,7 @@ function renderMappings() {
       (m) => `
     <div class="mapping-item" data-id="${m.id}">
       <div class="mapping-values">
-        <span class="mapping-real">${escapeHtml(m.real)}</span>
+        <span class="mapping-real">${m.category === 'password' ? '&bull;&bull;&bull;&bull;&bull;&bull;&bull;&bull;' : escapeHtml(m.real)}</span>
         &rarr;
         <span class="mapping-sub">${escapeHtml(m.substitute)}</span>
       </div>