UI: Standardize buttons and design system across the application

- Created a unified button system in style.css with primary, secondary, and icon variants.
- Standardized cards, inputs, and layout components for a more premium look.
- Refactored header, login, anime/series cards, and watchlist/downloads sections to use the new design system.
- Cleaned up inline styles and redundant local style blocks in templates.
- Updated JS-generated buttons to follow the new global styling.

static/js/series-search.js

@@ -54,10 +54,10 @@ async function handleSeriesSearch() {
                             </div>
                         ` : ''}
                         <div class="anime-card-actions">
-                            <button class="btn-secondary btn-small" onclick="window.open('${escapeHtml(s.url)}', '_blank')">
+                            <button class="btn btn-secondary btn-small" onclick="window.open('${escapeHtml(s.url)}', '_blank')">
                                 🔗 Voir sur FS7
                             </button>
-                            <button class="btn-primary btn-small" onclick="loadSeriesEpisodesDirect('${escapeHtml(s.url)}', '${escapeHtml(s.title)}')">
+                            <button class="btn btn-primary btn-small" onclick="loadSeriesEpisodesDirect('${escapeHtml(s.url)}', '${escapeHtml(s.title)}')">
                                 📥 Voir les épisodes
                             </button>
                         </div>
@@ -111,7 +111,7 @@ async function loadSeriesEpisodesDirect(url, title) {
                             <option value="${escapeHtml(ep.url)}">Épisode ${escapeHtml(ep.episode)}</option>
                         `).join('')}
                     </select>
-                    <button class="btn-primary" style="margin-top: 10px; width: 100%;" onclick="downloadSeriesEpisode('${escapeHtml(url)}', '${escapeHtml(title)}')">
+                    <button class="btn btn-primary" style="margin-top: 10px; width: 100%;" onclick="downloadSeriesEpisode('${escapeHtml(url)}', '${escapeHtml(title)}')">
                         <svg fill="none" stroke="currentColor" viewBox="0 0 24 24" style="width:14px;height:14px;margin-right:4px;">
                             <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 16v1a3 3 0 003 3h10a3 3 0 003-3v-1m-4-4l-4 4m0 0l-4-4m4 4V4"></path>
                         </svg>