feat: Add comprehensive backend validation, toast notifications, and Tokyo Night theme

- Backend: Add server-side validation with go-playground/validator/v10
- Frontend: Add toast notifications for API errors with @zerodevx/svelte-toast
- UI: Complete Tokyo Night theme redesign with modern animations
- Docs: Update COMPLETE_DOCUMENTATION.md and README.md with all enhancements
- Validation: Email format, password strength, and input sanitization
- UX: Real-time error feedback, loading states, and responsive design

backend/static/_app/immutable/assets/0.DLollcCP.css

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+.static{position:static}.container{width:100%}.mx-auto{margin-inline:auto}.line-clamp-2{-webkit-line-clamp:2;-webkit-box-orient:vertical;display:-webkit-box;overflow:hidden}.line-clamp-3{-webkit-line-clamp:3;-webkit-box-orient:vertical;display:-webkit-box;overflow:hidden}.contents{display:contents}.flex{display:flex}.grid{display:grid}.aspect-\[3\/4\]{aspect-ratio:3/4}.h-full{height:100%}.w-full{width:100%}.grid-cols-1{grid-template-columns:repeat(1,minmax(0,1fr))}.items-center{align-items:center}.justify-center{justify-content:center}.overflow-hidden{overflow:hidden}.object-cover{-o-object-fit:cover;object-fit:cover}.text-center{text-align:center}.transition-shadow{transition-property:box-shadow;transition-timing-function:var(--tw-ease,ease);transition-duration:var(--tw-duration,0s)}

backend/static/_app/immutable/assets/2.D5pqjzFo.css

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+.line-clamp-2.svelte-1uha8ag{display:-webkit-box;-webkit-line-clamp:2;-webkit-box-orient:vertical;overflow:hidden}.line-clamp-3.svelte-1uha8ag{display:-webkit-box;-webkit-line-clamp:3;-webkit-box-orient:vertical;overflow:hidden}