PWA offline error page + login page cleanup (#327)

* Add friendly PWA offline error page

When the PWA fails to connect to the server, users now see a branded
offline page with a friendly "technical difficulties" message, the
app logo, and a reload button. The page automatically attempts to
reload when connectivity is restored.

Changes:
- Created public/offline.html with branded offline experience
- Updated service worker to cache and serve offline page on network failures
- Added service worker registration in application.js
- Service worker now handles navigation requests with offline fallback

* Extract PWA offline logo to separate cached asset

Move the inline SVG logo from offline.html to a separate file at
public/logo-offline.svg. This makes the logo asset easily identifiable
and maintainable, as it may diverge from other logo versions in the future.

Changes:
- Created public/logo-offline.svg with the offline page logo
- Updated service worker to cache logo as part of OFFLINE_ASSETS array
- Updated fetch handler to serve cached offline assets
- Updated offline.html to reference logo file instead of inline SVG

* Update offline message for better readability

Signed-off-by: Juan José Mata <juanjo.mata@gmail.com>

* CodeRabbit comments

* Keep 40x and 50x flowing

* Dark mode

* Logo tweaks

* Login/sign up cleanup

---------

Signed-off-by: Juan José Mata <juanjo.mata@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>

app/views/layouts/auth.html.erb

@@ -4,12 +4,9 @@
       <div class="grow flex flex-col justify-center">
         <div class="sm:mx-auto sm:w-full sm:max-w-md">
           <div class="flex justify-center mt-2 md:mb-6">
-            <%= image_tag "logo-color.png", class: "w-16 mb-6" %>
+            <%= image_tag "logomark.svg", class: "w-16 mb-6" %>
           </div>
           <div class="space-y-2">
-            <h2 class="text-3xl font-medium text-primary text-center">
-              <%= content_for?(:header_title) ? yield(:header_title).html_safe : t(".your_account") %>
-            </h2>
             <% if (controller_name == "sessions" && action_name == "new") || (controller_name == "registrations" && action_name == "new") %>
               <div class="space-y-3 md:hidden w-full my-4">
                 <div class="bg-surface-inset rounded-lg p-1 flex">