Fix --sp-5 references to a nonexistent spacing token (#46 follow-up)

The spacing scale (tokens.css) jumps from --sp-4 (1rem) to --sp-6
(1.5rem) -- there's no --sp-5. Three places referenced it anyway, so
those margin declarations were invalid at compute time and silently
resolved to 0:

- ThemeBuilderModal.css's .custom-theme-preview margin-bottom -- the bug
  the user actually noticed, as a ~0px gap between the preview mockup and
  the color fields below it that read as a visual overlap.
- Modal.css's .modal-divider -- a shorthand `margin: var(--sp-5) 0
  var(--sp-4)`, where one invalid value invalidates the whole
  declaration, so every <hr class="modal-divider"> (ProfileModal's
  section separators, etc.) has had zero margin on both sides.
- AdminPage.css's .admin-invite-list margin-bottom.

All three now use --sp-6, matching the spacing tier that was clearly
intended.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/Modal.css

@@ -91,7 +91,7 @@
 .modal-divider {
   border: none;
   border-top: 1px solid var(--ds-border);
-  margin: var(--sp-5) 0 var(--sp-4);
+  margin: var(--sp-6) 0 var(--sp-4);
 }
 
 .theme-swatch-grid {

frontend/src/components/ThemeBuilderModal.css

@@ -5,7 +5,7 @@
 
 .theme-builder-modal .custom-theme-preview {
   margin-top: var(--sp-2);
-  margin-bottom: var(--sp-5);
+  margin-bottom: var(--sp-6);
 }
 
 .theme-builder-lower {

frontend/src/pages/AdminPage.css

@@ -168,7 +168,7 @@
   border: 1px solid var(--ds-border);
   border-radius: var(--radius);
   padding: var(--sp-3) var(--sp-4);
-  margin-bottom: var(--sp-5);
+  margin-bottom: var(--sp-6);
 }
 
 .admin-invite-list-label {