feat: add Unstoppable Wallet to ecosystem

[joshuakrueger-dfx]

Add Unstoppable Wallet (Horizontal Systems) to the ecosystem page. It ships native OpenCryptoPay support on iOS and Android, so it belongs in the payment-app listing next to the other OCP-capable wallets.

Changes

• ecosystem.html — one card in the "Show all" tab and one in the "Payment Apps" tab, both directly after the urble card. Structure copied verbatim from the neighbouring Aqua card, including the Webflow data-w-id attributes; only target URL, icon, name, description and link label differ. Badge is Payment App, the badge every other wallet on the page carries (21 of them before this change).
• images/Ecosystem-Icon-unstoppable.svg — new, built from the official mark on unstoppable.money and matched to the proportions of the official app icon (unstop_1024.png): the U spans 34.7% of the tile width and sits flush with the top edge, on the brand gradient. 42×43 with a 4px rounded rect, the same shape the sibling icons use.

Verified in the browser

Served from the branch and checked with a real browser, not by reading the diff:

• the card renders in "Show all" (position 29 of 32) and in "Payment Apps" (position 23 of 23), in both cases directly after urble
• the card is present in exactly those two panes and in no other tab
• the icon loads at its natural 42×43 with no broken image
• no new console errors

Scope

The wallet's supported chains are listed in the description text only; the machine-readable entry lives in the API (wallet_app), which is a separate change.