fix(ext/login): constrain lower form sections to .form-grid envelope (P3)

Notes, custom-fields disclosure, attachments disclosure, and form-actions
in fullscreen logins now sit inside a .form-lower wrapper with the same
max-width: 960px; margin: 0 auto envelope as .form-grid above. Removes
the visual rhythm break at the 2-col -> full-width transition.

Popup keeps its current single-column behavior (gated on surface flag).

extension/src/popup/components/types/login.ts

@@ -357,6 +357,7 @@ export function renderForm(
 
       ${sectionsHtml}
 
+      <div class="${surface === 'fullscreen' ? 'form-lower' : ''}">
         <div class="form-group">
           <div class="notes-with-toggle">
             <label class="label" for="f-notes" style="margin:0;flex:1;">notes</label>
@@ -372,6 +373,7 @@ export function renderForm(
           <button class="btn btn-primary" id="save-btn">${state.loading ? '<span class="spinner"></span>' : 'save'}</button>
         </div>
       </div>
+    </div>
   `;
 
   const rerender = (): void => {

extension/src/vault/vault.css

@@ -1626,6 +1626,20 @@ textarea {
 @media (max-width: 720px) {
   .form-grid { grid-template-columns: 1fr; }
 }
+
+/* P3: lower form sections constrained to the same envelope as .form-grid.
+   Gated on surface === 'fullscreen' in login.ts; popup unaffected. */
+.form-lower {
+  max-width: 960px;
+  margin: 0 auto;
+}
+.form-lower > .form-group,
+.form-lower > .disclosure,
+.form-lower > .attachments-disclosure,
+.form-lower > .form-actions {
+  width: 100%;
+}
+
 .form-col {
   padding: 14px 16px;
 }