Show offline mode UI: hide online-only nav items, show offline label

When offline, hide Files and Admin from the bottom nav and display
"Offline Mode" next to the status dot. The SW offline fallback page
also only shows Capture and Queue nav items. Online-only pages
(browser, admin, reviewer) are never cached and go straight to the
offline fallback when the network is unavailable.

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

app/static/css/style.css

@@ -766,3 +766,10 @@ textarea,
 select {
     transition-property: all;
 }
+
+/* Offline mode label */
+.offline-label {
+    font-size: 0.75rem;
+    color: var(--offline, #888);
+    font-weight: 600;
+}