web: harvest the v1 login UI and rewire it to the facade

[FSM1]

Goal

Bring back the v1 login UI and rewire it to the facade.

Harvest / create

Harvest from branch v1:

• components/auth/{EmailLoginForm,GoogleLoginButton,WalletLoginButton,LogoutButton}.tsx
• routes/Login.tsx
• components/{MatrixBackground,StagingBanner}.tsx
• lib/web3auth/*, lib/wagmi/*, login styles.

Rewire

• New useAuth drives Core Kit login but routes the secret through the P2 handoff (web: land the useSyncExternalStore snapshot adapter and Core Kit login handoff #803), never JS IPNS derivation.
• SIWE signature -> facade.siweLogin(message,signature).
• Logout -> facade.logout() + worker teardown.

Engine seams

start, siweLogin, logout.

Tests

Each login method dispatches the handoff; SIWE routes to facade.siweLogin; logout tears down.

Gate

web unit.

Depends on

#803.

Blocks

#809, #910, #913, #914.

Part of #642

[FSM1]

Landed in #911. The SIWE nonce could not be routed below the facade in this slice — the engine exposes no challenge command — so #910 tracks moving it there and settling whether SIWE belongs on the cold login page at all.

[FSM1]

Follow-ups from the review gates on #911: #913 for the Core Kit storage scope, and #914 for deriving auth state from the engine instead of the tab-local store.