Add a warning about missing Application Cache support to the server home page

src/pyj/book_list/constants.pyj

@@ -6,3 +6,8 @@ from __python__ import hash_literals, bound_methods
 book_list_container_id = 'book-list-container'
 read_book_container_id = 'read-book-container'
 INIT_ENDPOINT = 'interface-data/init'
+
+def has_offline_support(val):
+    if val is not undefined:
+        has_offline_support.ans = val
+    return has_offline_support.ans

src/pyj/book_list/home.pyj

@@ -6,13 +6,14 @@ from elementmaker import E
 from gettext import gettext as _
 
 from ajax import ajax_send
+from book_list.constants import has_offline_support
 from book_list.cover_grid import BORDER_RADIUS
 from book_list.globals import get_db
 from book_list.library_data import last_virtual_library_for, sync_library_books
 from book_list.router import open_book, update_window_title
 from book_list.top_bar import add_button, create_top_bar
 from book_list.ui import set_default_panel_handler, show_panel
-from dom import add_extra_css, build_rule, clear, ensure_id, unique_id, set_css
+from dom import add_extra_css, build_rule, clear, ensure_id, set_css, unique_id
 from modals import create_custom_dialog
 from session import get_device_uuid, get_interface_data
 from utils import conditional_timeout, safe_set_inner_html, username_key
@@ -256,5 +257,12 @@ def init(container_id):
             cl.lastChild.style.margin = '1ex 1rem'
             cl.lastChild.dataset.lid = library_id
 
+    if not has_offline_support():
+        container.appendChild(E.div(
+            style='margin-top: 1rem; padding: 1rem; border-top: solid 1px currentColor',
+            _('WARNING: This browser does not support Application Cache so offline access'
+              ' to calibre will not work. Some browsers only enable Application Cache over HTTPS'
+              ' connections, so you may have to setup HTTPS for your calibre server.')))
+
 
 set_default_panel_handler(init)

src/pyj/srv.pyj

@@ -10,6 +10,7 @@ from ajax import ajax, console_print, set_default_timeout
 from autoreload import create_auto_reload_watcher
 from book_list.globals import main_js
 from book_list.main import main
+from book_list.constants import has_offline_support
 
 is_running_in_iframe = False  # Changed before script is loaded in the iframe
 autoreload_enabled = False
@@ -50,7 +51,9 @@ else:
         window.applicationCache.addEventListener('updateready', on_appcache_ready)
         if window.applicationCache.status is window.applicationCache.UPDATEREADY:
             on_appcache_ready()
+        has_offline_support(True)
     else:
+        has_offline_support(False)
         console.warn("Your browser does not have a working Application Cache, offline access will not work")
     script = document.currentScript or document.scripts[0]
     main_js(script.textContent)