Celebration UI, import fixes & CI workflow

[w3spi5]

Summary

• 🎆 Add celebration effects (fireworks + confetti) on successful import
• 🔧 Fix 99.5% progress bug + SSE completion redirect
• 🎨 Enhanced dark mode styling + animated favicon + skeleton loaders
• ⚙️ Fix CI workflow for protected branch compatibility

New Features

• Fireworks: 20-second particle display with trails, glow, and gradients
• Confetti: Infinite falling 3D pieces with realistic air physics
• Animated Favicon: Bouncing arrow during active import
• Skeleton Loaders: Shimmer animation for loading states

Bug Fixes

• Progress bar correctly reaches 100% on completion
• Success message displays in-page (no redirect to error)
• French localization: "Import terminé !"

UI Improvements

• .stat-card-success with gradient backgrounds and glowing borders
• .progress-bar-complete with green shimmer and pulsing glow
• Enhanced dark mode contrast

CI Fix

• build-assets.yml now only auto-commits on develop branch
• PR to main triggers build verification without push attempt
• Compatible with branch protection rules

Files Changed

• src/Services/AjaxService.php - SSE handlers, success display, 100% fix
• assets/src/js/celebration.js - Fireworks + confetti effects
• assets/src/js/favicon-animator.js - Animated favicon
• assets/src/css/tailwind.css - Animations and stat card styles
• templates/import.php - Script includes, skeleton loaders
• .github/workflows/build-assets.yml - Branch protection fix