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/15-ui-ux-best-practices.md

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+
+### Consistency
+
+- **Visual Consistency**: Same colors, typography, spacing throughout
+- **Functional Consistency**: Same actions produce same results
+- **External Consistency**: Follow platform conventions and user expectations
+
+### Feedback
+
+- **Immediate**: Response within 100ms feels instantaneous
+- **Visual**: Button state changes, loading indicators, success/error states
+- **Audio**: Subtle sounds for significant actions
+- **Haptic**: Vibration feedback on mobile for confirmations
+
+### Error Prevention
+
+- **Constraints**: Limit input to valid values (date pickers, dropdowns)
+- **Validation**: Real-time validation with clear messages
+- **Confirmations**: Confirm destructive actions before executing
+- **Undo**: Allow reversal of actions when possible
+
+### User Control
+
+- **Freedom**: Easy navigation, clear exit paths
+- **Undo/Redo**: Reverse actions easily
+- **Cancel**: Abort processes without penalty
+- **Customization**: User preferences and settings
+
+### Recognition Over Recall
+
+- **Visible Options**: Display available actions and choices
+- **Autocomplete**: Suggest based on history and context
+- **Recently Used**: Surface frequently accessed items
+- **Smart Defaults**: Pre-populate with sensible values
+
+---