docs: add comprehensive frontend landing page plan and download design skills

Add detailed landing page development plan in docs/frontend_landing_plan.md:
- Complete landing page structure (Hero, Problem/Solution, Features, Demo, CTA)
- Design guidelines from downloaded skills (typography, color, motion, composition)
- Security considerations (XSS prevention, input sanitization, CSP)
- Performance targets (LCP <2.5s, bundle <150KB, Lighthouse >90)
- Responsiveness and accessibility requirements (WCAG 2.1 AA)
- Success KPIs and monitoring setup
- 3-week development timeline with daily tasks
- Definition of Done checklist

Download 10+ frontend/UI/UX skills via universal-skills-manager:
- frontend-ui-ux: UI/UX design without mockups
- frontend-design-guidelines: Production-grade interface guidelines
- frontend-developer: React best practices (40+ rules)
- frontend-engineer: Next.js 14 App Router patterns
- ui-ux-master: Comprehensive design systems and accessibility
- ui-ux-systems-designer: Information architecture and interaction
- ui-ux-design-user-experience: Platform-specific guidelines
- Plus additional reference materials and validation scripts

Configure universal-skills MCP with SkillsMP API key for curated skill access.

Safety first: All skills validated before installation, no project code modified.

Refs: Universal Skills Manager (github:jacob-bd/universal-skills-manager)
Next: Begin Sprint 3 landing page development

.opencode/skills/ui-ux-design-user-experience/references/9-usability-principles.md

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+
+### Nielsen's 10 Usability Heuristics
+
+1. **Visibility of System Status**: Keep users informed with appropriate feedback
+2. **Match Between System and Real World**: Use familiar language and concepts
+3. **User Control and Freedom**: Provide undo, redo, and easy exits
+4. **Consistency and Standards**: Follow platform conventions
+5. **Error Prevention**: Design to prevent errors before they occur
+6. **Recognition Rather Than Recall**: Make options and actions visible
+7. **Flexibility and Efficiency of Use**: Accelerators for expert users
+8. **Aesthetic and Minimalist Design**: Remove irrelevant information
+9. **Help Users Recognize, Diagnose, and Recover from Errors**: Clear error messages
+10. **Help and Documentation**: Provide searchable, task-focused help
+
+### Cognitive Load
+
+**What Is It?**
+The mental effort required to use an interface. High cognitive load leads to user frustration and errors.
+
+**Reducing Cognitive Load:**
+- Chunk information into digestible pieces
+- Use progressive disclosure
+- Provide clear visual hierarchy
+- Minimize choices (paradox of choice)
+- Use familiar patterns and conventions
+- Reduce required memory with visual cues
+
+### Fitts's Law
+
+**What Is It?**
+The time to reach a target is proportional to distance and inversely proportional to target size.
+
+**UI Implications:**
+- Make important buttons large and easy to reach
+- Place frequently used actions in accessible areas
+- Increase touch targets for mobile (minimum 44x44px)
+- Use edge and corner positions for key actions (infinite depth)
+
+### Hick's Law
+
+**What Is It?**
+Decision time increases logarithmically with the number of choices.
+
+**UI Implications:**
+- Limit options to reduce decision time
+- Categorize and group related options
+- Use progressive disclosure for complex choices
+- Highlight recommended options
+
+### Gestalt Principles
+
+- **Proximity**: Elements close together are perceived as grouped
+- **Similarity**: Similar elements are perceived as related
+- **Closure**: Mind completes incomplete shapes
+- **Continuity**: Eyes follow continuous lines and patterns
+- **Figure-Ground**: Perception of foreground and background
+- **Common Fate**: Elements moving together are perceived as grouped
+
+---