perf: keep Slise banner mounted across navigation + gapless per-page …

[devmildfire]

Keep Slise banner mounted across navigation + gapless per-page refresh

Background

PR #686 fixed the cold-load delay (preconnect + preload of embed.js, and
syncing on the script load event instead of a one-shot typeof check). After
that merged you noted:

"it's better now, but there still a delayed refresh during page switching."

This PR fixes that remaining navigation delay.

What was wrong (measured on production ping.pub)

<AdBanner> lived inside <RouterView><Transition mode="out-in">, so it was
destroyed and recreated on every client-side navigation. On each remount
Slise blanks the <ins> and refetches a creative, so the banner went blank and
only repainted once the (often heavy) destination page finished mounting.

Timing click → banner visible across 37 real navigations:

scenario	median	max
chain routes (cosmos, 7 routes)	~500ms	~2665ms
wallet routes (warm)	~150ms	~193ms

The delay tracked page weight — the blank banner competed with the route
component for the main thread, so /cosmos/uptime was consistently ~2.5s blank.
It was not a queue leak (window.adsbyslise stays length 1) and not a
CSS transition.

The fix

1. Persist the banner. <AdBanner> is now a sibling above <RouterView>
   in DefaultLayout.vue, so it mounts once and stays mounted across
   navigations — never destroyed, so it never blanks.

2. Refresh gaplessly per navigation. AdBanner watches route.fullPath.
   On each navigation it removes data-ad-loaded from its <ins> without
   clearing the existing creative, then calls window.adsbyslisesync(). Slise
   refetches and swaps the new creative in only when ready, so the previous
   ad stays on screen the whole time — no blank gap.

   (Re-calling adsbyslisesync() on an <ins> that still has data-ad-loaded
   does nothing — Slise skips loaded slots — which is why the flag must be
   dropped first. Verified against the live embed.)

Result

• No blank gap on navigation — old creative stays visible while the next
  loads in the background.
• A fresh creative (impression) per page view is preserved.
• Behaviour is now independent of destination-page weight.

Trade-off / rollback

Keeps a per-navigation ad refresh (every page view = fresh impression). If
that is not wanted, delete the route.fullPath watcher in AdBanner.vue and
the banner stays a single persistent ad — still gap-free, just no rotation.