[Wave 200pts] Add a security review checklist doc for the withdrawal/deposit flows

[portableDD]

Summary

No security-focused review checklist exists specifically for the withdrawal and deposit flows, despite these being the app's highest-stakes surfaces (they handle real fund-movement instructions) — future changes to these flows currently have no dedicated checklist to guide reviewer attention toward the risks specific to this area.

Why This Matters

A short, dedicated checklist for the highest-risk flows in the app (address validation, amount validation, confirmation-step integrity, fee transparency) gives reviewers a consistent, documented set of things to specifically verify on every future change to these flows, rather than relying on each reviewer's individual judgment.

What Needs to Be Done

• Write a concise security-review checklist covering the withdrawal and deposit flows specifically (address/format validation, amount-limit enforcement, confirmation-step correctness, fee disclosure accuracy, and any other risks identified elsewhere in this batch)
• Reference relevant issues from this batch (hardcoded deposit address, WithdrawalReview fee calculation, input length limits) as concrete examples the checklist is meant to catch in the future
• Link the checklist from the PR template or CONTRIBUTING documentation so it's actually surfaced when a relevant change is proposed

Key Files

• docs/security-checklist.md
• components/dashboard/withdrawal/
• components/dashboard/deposit.tsx

Acceptance Criteria

• A security-review checklist document exists specifically covering the withdrawal and deposit flows
• It's linked from a place reviewers would actually encounter it when reviewing a relevant change
• Complexity: 200 points