Add Stellar Expert activity widget to landing page

[Sikkra]

Summary

• add an on-chain activity section to the landing page with a fixed-height Stellar Expert embed
• link the widget to the known deployed testnet leveraged USDC strategy contract
• add a timeout/error fallback that hides the iframe and keeps a direct Stellar Expert link available

Note: the mainnet vault ID is still blank in frontend/src/defindex.ts, so this uses the configured testnet strategy contract rather than implying a mainnet deployment.

Verification

• git diff --check
• PowerShell static check for activity nav, section, iframe URL, fallback timeout, and fixed desktop/mobile heights
• node -e "const fs=require('fs'); const html=fs.readFileSync('landing/index.html','utf8'); const scripts=[...html.matchAll(/<script>([\s\S]*?)<\/script>/g)].map(m=>m[1]); for (const script of scripts) new Function(script); console.log('inline_scripts_valid=' + scripts.length);"
• npx --yes html-validate@latest --rule void-style:off landing\index.html

Browser note: the Codex in-app browser blocked direct local file:// navigation by policy, so I did not use a browser workaround for the static page.

Closes #91