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/sessions/new.html.erb

@@ -1,7 +1,3 @@
-<%
-  header_title t(".title")
-%>
-
 <% if @prefill_demo_credentials %>
   <div class="mb-4 rounded-lg border bg-blue-50 text-blue-700 border-blue-200 theme-dark:bg-blue-900/20 theme-dark:text-blue-400 theme-dark:border-blue-800 p-4" role="status" aria-live="polite">
     <div class="flex items-start gap-3">