Offline CUPS from the normal menu, local-file sites, build-dep install

- CUPS (Addons -> CUPS Printing, from ./install.sh or the Web UI) now
  installs from the offline bundle by itself when the kiosk can't reach
  the Ubuntu archive: lib/config.sh's run_with_offline_fallback switches
  apt to the bundle (kiosk-offline-apt on) around the install and back
  afterwards. No-op on manual installs or when online.
- Fix cups_is_installed reporting "not installed" for an installed CUPS:
  `dpkg -l | grep -q` under install.sh's pipefail fails when grep exits
  early and dpkg takes SIGPIPE. Uses dpkg-query on the one package now.
- Sites: an absolute path (/home/kiosk/docs/menu.pdf) is accepted and
  turned into a percent-encoded file:// URL, identically in the Sites
  menu (sites_parse_url) and the Web UI (parseUrl, with tests). Electron
  42 shows local HTML, PDFs (built-in viewer) and images as-is.
- iso/build-iso.sh checks its build tools (plus the --offline ones) and
  offers to `sudo apt install` what's missing; --install-deps does it
  without asking. --offline now refuses non-amd64 hosts up front.

Tested: offline chroot of the 26.04.1 server filesystem - action_install_cups
installed CUPS + printer-driver-all/hplip/gutenprint from the bundle with
no network and left apt back on normal sources; webui test suite passes;
Electron 42 rendered a file:// PDF and PNG with spaces/# in the path;
build-iso.sh stopped with the install command when apt-utils was missing
and no TTY, and installed it and built the ISO with --install-deps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cjz8a58eB5P9sUweqd3NHm

menus/sites.sh

@@ -18,11 +18,23 @@ SITE_EDIT_IDX=""
 ################################################################################
 
 # Normalize whatever the user typed into a URL, same rules the old
-# installer used: bare host -> https://, bare IP -> http://.
+# installer used: bare host -> https://, bare IP -> http://. Plus local
+# files on the kiosk itself (pages, PDFs, images): an absolute path ->
+# file://, percent-encoded the same way webui/server.js's parseUrl does.
 sites_parse_url() {
     local raw="$1"
-    if [[ "$raw" =~ ^https?:// ]]; then
+    if [[ "$raw" =~ ^(https?|file):// ]]; then
         echo "$raw"
+    elif [[ "$raw" == /* ]]; then
+        local LC_ALL=C out="" c i
+        for (( i = 0; i < ${#raw}; i++ )); do
+            c="${raw:i:1}"
+            case "$c" in
+                [A-Za-z0-9._~/!\'\(\)*-]) out+="$c" ;;
+                *) out+="$(printf '%%%02X' "'$c")" ;;
+            esac
+        done
+        echo "file://$out"
     elif [[ "$raw" =~ ^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+ ]]; then
         echo "http://${raw}"
     else