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/anime_card.html

@@ -16,7 +16,7 @@
 
         <div class="anime-overlay">
             <div class="overlay-buttons">
-                <button class="btn-circle" 
+                <button class="btn btn-primary btn-circle" 
                         hx-get="/api/anime/episodes?url={{ anime.url | urlencode }}" 
                         hx-target="#player-container"
                         hx-swap="innerHTML"
@@ -38,13 +38,13 @@
         </div>
         
         <div class="anime-card-buttons">
-            <button class="btn-card btn-watch" 
+            <button class="btn btn-primary btn-small" 
                     hx-get="/api/anime/episodes?url={{ anime.url | urlencode }}" 
                     hx-target="#player-container"
                     hx-swap="innerHTML">
                 <i class="fas fa-eye"></i> <span>Regarder</span>
             </button>
-            <button class="btn-card btn-download" 
+            <button class="btn btn-secondary btn-small" 
                     hx-get="/api/anime/episodes?url={{ anime.url | urlencode }}" 
                     hx-target="#player-container"
                     hx-swap="innerHTML">
@@ -53,7 +53,7 @@
         </div>
 
         {% if not in_watchlist %}
-        <button class="btn-add-watchlist"
+        <button class="btn btn-secondary btn-small btn-block"
                 hx-post="/api/watchlist"
                 hx-vals='{"anime_url": "{{ anime.url }}", "anime_title": "{{ anime.title }}", "provider_id": "{{ anime.provider_id }}"}'
                 hx-swap="none"
@@ -61,7 +61,7 @@
             <i class="fas fa-plus"></i> Watchlist
         </button>
         {% else %}
-        <button class="btn-add-watchlist followed" disabled>
+        <button class="btn btn-secondary btn-small btn-block followed" disabled>
             <i class="fas fa-check"></i> Suivi
         </button>
         {% endif %}