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

webui/test/api.test.js

@@ -80,6 +80,8 @@ async function main() {
                     { url: 'example.com', duration: 30, name: 'bare host' },
                     { url: '192.168.1.50', duration: 30, name: 'bare ip' },
                     { url: 'https://already.example.com', duration: 30, name: 'already a url' },
+                    { url: '/home/kiosk/docs/menu #2.pdf', duration: 30, name: 'local path' },
+                    { url: 'file:///srv/kiosk/index.html', duration: 30, name: 'already a file url' },
                 ],
             }),
         });
@@ -88,6 +90,8 @@ async function main() {
         assert.strictEqual(body.tabs[0].url, 'https://example.com');
         assert.strictEqual(body.tabs[1].url, 'http://192.168.1.50');
         assert.strictEqual(body.tabs[2].url, 'https://already.example.com');
+        assert.strictEqual(body.tabs[3].url, 'file:///home/kiosk/docs/menu%20%232.pdf');
+        assert.strictEqual(body.tabs[4].url, 'file:///srv/kiosk/index.html');
     });
 
     await check('PUT rejects homeTabIndex out of range', async () => {