Handle wallet disconnect during unlock

[Hibhee01]

Overview

Handles wallet disconnects during prompt unlocks by stopping the unlock flow before the unlock API is called and surfacing a contextual reconnect prompt to the buyer.

Related Issue

Closes #428

Changes

• Added a wallet connection guard to the prompt unlock helper so disconnects after challenge creation or message signing are detected before decrypting.
• Updated the prompt modal to show a prompt-specific reconnect message and reconnect action when the wallet drops mid-unlock.
• Tracked wallet disconnect unlock failures with a dedicated wallet_disconnected reason code.
• Added regression coverage for the guarded unlock helper and reconnect prompt.

Verification Results

• git diff --check - passed.
• corepack.cmd yarn install --immutable - failed before tests because Yarn reports the checked-in lockfile would be modified.
• corepack.cmd yarn vitest run src/lib/prompts/unlock.test.ts src/test/wallet/PurchaseButton.test.tsx - could not run because Yarn has no node_modules state file without a successful install.

Acceptance Criteria

Criteria	Status
Detect wallet disconnect during unlock	Complete
Prompt buyer to reconnect with prompt context	Complete
Avoid silent generic unlock failure	Complete
Add focused regression coverage	Complete

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