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.

templates/components/home_section.html

@@ -7,7 +7,7 @@
     <div class="section-container">
         <div class="section-header">
             <h2>🎯 Recommandé pour vous</h2>
-            <button class="btn-secondary btn-small" 
+            <button class="btn btn-secondary btn-small" 
                     hx-get="/api/recommendations" 
                     hx-target="#recommendationsList">
                 <i class="fas fa-sync-alt"></i> Actualiser
@@ -25,7 +25,7 @@
     <div class="section-container">
         <div class="section-header">
             <h2>🔥 Dernières sorties</h2>
-            <button class="btn-secondary btn-small" 
+            <button class="btn btn-secondary btn-small" 
                     hx-get="/api/releases/latest" 
                     hx-target="#releasesList">
                 <i class="fas fa-sync-alt"></i> Actualiser