mobile: implement onboarding create flow (backlog #25)

[popdadyrisky]

Implement mobile onboarding create flow that:
Registers a WebAuthn passkey via the Invisible Wallet SDK
Derives a deterministic fee-payer Keypair from the passkey credential ID (HKDF → Ed25519)
Funds the fee-payer via Friendbot (testnet) or verifies mainnet funding
Deploys the wallet contract through the factory using wallet.deploy()
Writes session state (invisible_wallet_address, veil_signer_secret) to sessionStorage and navigates to /dashboard
Files:
frontend/mobile/app/(onboarding)/create.tsx
Acceptance:
WebAuthn credential created → wallet contract deployed → session set → dashboard landing (see QA steps)
Notes:
The mobile flow keeps the signer secret ephemeral until after deploy (opt-in reveal). Web onboarding persists the secret prior to deploy; consider aligning behavior if desired.

closes #453

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[Miracle656]

Thanks! One thing to reconcile before this lands: main already has a create-wallet flow at frontend/mobile/app/create-wallet.tsx, and this adds a separate app/(onboarding)/create.tsx — so the app would have two create screens on different routes. Could you rebase onto main and either fold this into the existing create-wallet.tsx or clearly replace it (and update the navigation), rather than adding a parallel route? Then I'll re-review.