Add "Try Franklin" — in-browser wallet-paid playground (/try)

[KillerQueen-Z]

What

Adds a Try Franklin nav item linking to /try — a web preview of Franklin, styled to match the site (warm-minimal + gold). Users connect a browser-extension wallet and chat with frontier AI models, paying per message in USDC via x402. No account, no subscription.

Modeled on BlockRun's video-model experience page; reuses BlockRun's existing x402 API instead of standing up a new backend.

How it works

browser  →  franklin.run /api/blockrun/...  →  blockrun.ai /api/...
  (wallet signs x402)         (transparent proxy)        (runs model, settles USDC)

• /api/blockrun/[...path] — transparent proxy to blockrun.ai. Relays the X-Payment request header upstream and the payment-required response header back, so the browser stays same-origin (no CORS, 402 visible to the client).
• Wallet / x402 stack ported from BlockRun's playground: wagmi-config, useUsdcBalance, useX402Payment (EIP-3009 TransferWithAuthorization signing).
• useFranklinChat — probe → 402 → wallet-sign → retry with X-Payment. Free models (nvidia/*) run with no wallet; paid models charge a few cents of USDC, settled on Base to BlockRun's configured wallet.

UI

• WalletProvider (wagmi + react-query) scoped to /try only — wallet stack stays out of every other page's bundle
• ConnectWallet chip with live USDC balance
• FranklinChat surface: model picker (free + paid), prompt suggestions, signing/thinking/error states

i18n

"Try Franklin" nav label localized across all 13 locales.

Dependencies

Adds wagmi, viem, @tanstack/react-query.

Verification

• npm run typecheck ✅
• npm run build ✅ (/try prerenders; wagmi/viem compile under Next 16 SSR)
• Verified locally: /try → 200, proxy /v1/models → 200, paid-model POST → 402 with payment-required header relayed correctly
• Not testable headlessly: the actual MetaMask signature popup (needs a browser + extension) — code path is the same proven flow as BlockRun's playground

Follow-ups (not in this PR)

• Image/video generation modes (chat is the primary surface here; images = single-shot, video needs polling — straightforward to add next using the same proxy + payment flow)
• Optional NEXT_PUBLIC_WALLETCONNECT_ID to enable mobile QR connect