perf(ads): preconnect + preload Slise embed so banners render as the page loads

[forge-welldone]

Problem

The Slise ad banner is blank for roughly a second after the page loads. The
cause is that the Slise embed script is only injected inside AdBanner's
onMounted, i.e. after the app bundle has downloaded, parsed, and mounted
the ad component — and it's requested over a cold connection with no resource
hints.

Measured on the live site (Performance API, homepage, fast connection):

Event	Time from navigation start
embed.js requested	~1013 ms
embed.js finished loading	~2022 ms
ad request POST /target/v1/native	~2078 ms
creative iframe painted	~2767 ms

So the ad script isn't even requested until ~1 s in, and nothing about the
ad load overlaps the app boot.

Changes

1. index.html — warm the Slise origin and preload the embed script

<link rel="preconnect" href="https://v1.slise.xyz" crossorigin />
<link rel="dns-prefetch" href="https://v1.slise.xyz" />
<link rel="preload" href="https://v1.slise.xyz/scripts/embed.js" as="script" />

This starts the ~21 KB embed.js download in parallel with the app bundle
(instead of ~1 s later) and warms DNS/TLS. embed.js is already allow-listed
in the existing CSP, so no CSP change is needed. The component still executes
the script only when an AdBanner mounts — the preload just makes the bytes
ready in advance, so no ad code runs on pages that don't render an ad.

2. AdBanner.vue — fill the slot as soon as the script is ready

The previous code did a one-shot typeof window.adsbyslisesync === 'function'
check immediately after appending the async script. On the first banner the
function isn't defined yet (the script is still loading), so the sync was
silently skipped and the first ad only filled once Slise's script finished and
ran its own auto-sync. This now waits for the script's load event, so the
first banner fills the moment the script is ready rather than depending on
Slise's internal 1-second location poll.

Verification

Built this branch and served the static output; the preload takes effect:

• embed.js initiatorType is now link (fetched by the preload, not the
  component), and its request start moves from ~1013 ms → ~14 ms after
  navigation — it now loads in parallel with the app bundle.

Type-check (vue-tsc --noEmit) and vite build both pass. The diff is two
files, no dependency or API changes.

Notes

Happy to also add a small loading skeleton inside the reserved ad box, or wire
distinct data-ad-slot values per placement (today every placement uses the
default desktop slot, so co-located banners share one slot) if you'd like —
kept this PR minimal and focused on the load-timing fix.