docs: add product and design documentation

PRODUCT.md

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+# MAL - Anime Streaming & Watchlist Service
+
+## Product Purpose
+Personal anime streaming service with watchlist management for me and my friends. Simple, functional, easy to navigate.
+
+## Users
+- **Primary**: Me (owner)
+- **Secondary**: Close friends with access
+- Technical comfort: Medium - comfortable with self-hosted services
+
+## Brand Tone
+- **Practical**: No fluff, just works
+- **Clean**: Minimal aesthetic, functional over decorative
+- **Personal**: Built for us, not for distribution
+
+## Design Principles
+1. **Easy to maneuver** - Clear navigation, predictable patterns
+2. **Content-first** - Anime artwork and details take precedence
+3. **Fast interactions** - HTMX for snappy partial updates
+4. **Dark theme default** - Easy on eyes for long viewing sessions
+
+## Anti-References
+- No anime list sites (MyAnimeList, AniList clones)
+- No generic SaaS dashboards
+- No "AI slop" aesthetics - gradient text, glassmorphism, hero metrics
+
+## Register
+**Product** - Design serves the utility, not the reverse.
+
+## Technical Stack
+- Go backend (HTMX for interactivity)
+- Tailwind CSS
+- No JavaScript framework - vanilla JS only