Playbook Website: Fixes for Dialog popup for VPN

[nidaqg]

What does this PR do?

Fixes the "VPN required" Playground dialog, which was unreliable in two ways after [PLAY-3203] shipped:

• False positive (Firefox-specific, off VPN): the reachability check used fetch(..., { mode: "no-cors" }), which resolves on any HTTP response — including a block/error page served for off-VPN traffic — so it couldn't tell a real block from success. That was also browser-dependent (Firefox's DNS handling could reach a different response than Chrome's), so the dialog showed correctly in Chrome but not Firefox, which just hit a raw 404 instead.
• False negative (all browsers, even on VPN): switching the check to an <img> load probe (onload/onerror instead of fetch) fixed the first bug, but the probe target, /favicon.ico, was never added to staging's route allow-list in routes.rb. Any unlisted path on staging falls through to its own catch-all, which redirects back to prod — so the probe was silently checking prod's favicon the whole time, always "succeeding" as a redirect, which made the dialog show up even while on VPN with no way to get to staging.

Fix:

• isStagingReachable() (siteNavigation.ts) now probes via an <img> element instead of a no-cors fetch — a non-image response (e.g. an HTML block page) correctly fails to decode and fires onerror, independent of browser-specific network/DNS behavior.
• Added GET /vpn_check.gif to staging's host-constrained routes, ahead of the catch-all, serving a tiny inline 1×1 GIF. Being explicitly allow-listed, it can never be swept into the redirect-to-prod fallback, so the probe now genuinely reflects staging's own reachability.

How to test?

1. Off VPN, in both Chrome and Firefox: open a kit's Playground tab and confirm the "VPN required" dialog shows consistently in both (no raw 404 in either).
2. On VPN, in any browser: open a kit's Playground tab and confirm it redirects straight to staging with no dialog.
3. curl -sI https://staging.playbook.powerapp.cloud/vpn_check.gif while on VPN — expect 200, Content-Type: image/gif, no redirect.

Checklist:

• LABELS bug
• DEPLOY milano label
• TESTS
• PLAYGROUND n/a (no kit/prop changes)
• SEMVER patch
• RC n/a

[nidaqg]

not doing this for now, going a different route