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/anime-details.js

@@ -268,10 +268,10 @@ function renderAnimeDetails(anime) {
                     ` : ''}
 
                     <div class="anime-details-actions">
-                        <a href="${escapeHtml(anime.url)}" target="_blank" class="btn-secondary btn-small">
+                        <a href="${escapeHtml(anime.url)}" target="_blank" class="btn btn-secondary btn-small">
                             🔗 Voir sur MAL
                         </a>
-                        <button onclick="searchAnimeOnProviders('${escapeHtml(anime.title)}')" class="btn-primary btn-small">
+                        <button onclick="searchAnimeOnProviders('${escapeHtml(anime.title)}')" class="btn btn-primary btn-small">
                             📥 Télécharger
                         </button>
                     </div>
@@ -291,7 +291,7 @@ function renderAnimeDetails(anime) {
                 <div class="anime-details-section">
                     <div style="display: flex; justify-content: space-between; align-items: center; margin-bottom: 15px;">
                         <h3 style="margin: 0;">📖 Synopsis</h3>
-                        <button onclick="translateSynopsis('${synopsisId}', this)" class="btn-secondary btn-small" style="font-size: 12px;">
+                        <button onclick="translateSynopsis('${synopsisId}', this)" class="btn btn-secondary btn-small" style="font-size: 12px;">
                             🌐 Traduire en français
                         </button>
                     </div>
@@ -405,7 +405,7 @@ function renderStreamingResult(result, query) {
                     ${episodes.length > 20 ? `<option disabled>... et ${episodes.length - 20} autres</option>` : ''}
                 </select>
 
-                <button class="btn-primary btn-small streaming-download-btn" onclick="downloadSelectedEpisode(this)">
+                <button class="btn btn-primary btn-small streaming-download-btn" onclick="downloadSelectedEpisode(this)">
                     📥 Télécharger
                 </button>
             </div>