[SUP-001] Support: Help Center — FAQ and documentation pages

## Overview
Build a static Help Center with FAQ articles, organized by category. This reduces support ticket volume by enabling self-service for common questions.

## Acceptance Criteria
- [ ] Route: `/help` shows category tiles: Orders, Returns, Payments, Account, Shipping
- [ ] Route: `/help/:category` lists articles in that category
- [ ] Route: `/help/:category/:article` renders a full help article (Markdown-rendered)
- [ ] Search within Help Center (client-side)
- [ ] "Was this helpful?" thumbs up/down feedback on each article
- [ ] Breadcrumbs and related articles section
- [ ] Link from the stub modal in the app to the relevant Help Center section
- [ ] At least 10 seed articles covering the most common questions

## Seed Article Topics
1. How do I track my order?
2. What is your return policy?
3. How do I cancel an order?
4. How do I use a coupon code?
5. I forgot my password — what do I do?
6. How do I update my shipping address?
7. When will my order ship?
8. What payment methods are accepted?
9. How long does a refund take?
10. How do I contact support?

## Depends On
- None — this can be built as static content independently