Replace any-typed walletApi with a typed Freighter API interface

[prodbycorne]

Problem

StellarWalletContext stores the Freighter module as walletApi: any and src/components/UnlockModal/UnlockModal.tsx passes it straight to unlockAssets as any. This bypasses TypeScript's type checking for every wallet call, making it easy to accidentally call a non-existent method or pass the wrong argument shape without a compile-time error.

const [walletApi, setWalletApi] = useState<any | null>(null);

Acceptance Criteria

• src/types/freighter.ts defines a FreighterApi interface mirroring the methods actually used: isConnected(), isAllowed(), requestAccess(), getAddress(), getNetworkDetails(), signTransaction(xdr: string, opts: { networkPassphrase: string }): Promise<{ signedTxXdr: string; error?: string }>
• StellarWalletContext types walletApi as FreighterApi | null — no more any
• StellarWalletContextValue.walletApi exposed to consumers is typed FreighterApi | null
• src/lib/soroban.ts all methods receiving walletApi parameter replace any with FreighterApi
• @stellar/freighter-api is imported via dynamic import as before, but cast to FreighterApi after the isConnected check (the import type is compatible)
• tsconfig.json has "strict": true; no @ts-ignore or as any remains in wallet-related files
• Running tsc --noEmit produces zero errors after the change

Relevant Files

• src/types/freighter.ts — create this file
• src/context/StellarWalletContext.tsx — replace any
• src/lib/soroban.ts — walletApi: any in method signatures
• src/components/UnlockModal/UnlockModal.tsx — prop type for walletApi