use LiveView navigation and add SVG fallback dimensions to fix FOUC

All internal links used plain <a href>, causing full page reloads
between pages in the same live_session. Changed to <.link navigate>
for client-side navigation across layout, product, cart, base, and
content components.

Added explicit width/height attributes to all SVG icons so they
render at sensible sizes before CSS loads on initial page visit.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/page_templates/checkout_success.html.heex

@@ -153,11 +153,11 @@
         </p>
 
         <p class="text-sm" style="color: var(--t-text-tertiary);">
-          If this page doesn't update, please <a
-            href="/contact"
+          If this page doesn't update, please <.link
+            navigate="/contact"
             class="underline"
             style="color: var(--t-accent);"
-          >contact us</a>.
+          >contact us</.link>.
         </p>
       </div>
     <% end %>