Show image thumbnails in the playlist Add Content card

The "Add Content" card listed available media as an icon plus a filename,
so the only way to tell two media items apart was to read near-identical
filenames. Images now render an actual thumbnail.

Previews are shown for every image type, not just JPEG: the library is a
mix of JPEG and PNG (the PDF/PPTX converter emits PNG), so a JPEG-only
rule would have left most items without a preview. Non-images keep the
existing emoji icon. A missing or unreadable file falls back to the icon
via onerror, and images are `loading="lazy"` since the list is scrollable.

This surfaced two layout bugs in the same card, both now fixed:

- .content-grid used bare `2fr 1fr`. A grid track has an automatic
  min-content floor, so the long unbreakable media filenames forced the
  track wider than the viewport — the page had a horizontal scrollbar
  (scrollWidth 1475 in a 1185px viewport). minmax(0, ...) fixes it.

- Long names were truncated with ellipsis, which hides the distinguishing
  part: media filenames differ at the END (..._slide_003.png). Names now
  wrap with overflow-wrap: anywhere instead.

The row is flex-wrap so that on a narrow card the Add button drops to its
own line rather than squeezing the name.

Verified: 5/5 items render an <img> whose URL returns HTTP 200 (including
a nested edited_media/ JPEG), no element overlaps at 1000-1500px, and dark
mode picks up the theme variables.

app/templates/content/manage_playlist_content.html

@@ -39,7 +39,11 @@
     
     .content-grid {
         display: grid;
-        grid-template-columns: 2fr 1fr;
+        /* minmax(0, …) is required: a bare `2fr 1fr` track has an automatic
+           min-content floor, so the long, unbreakable media filenames in the
+           Add Content card forced the track wider than the viewport and gave
+           the whole page a horizontal scrollbar. */
+        grid-template-columns: minmax(0, 2fr) minmax(0, 1fr);
         gap: 20px;
     }
     
@@ -87,6 +91,56 @@
         display: flex;
         justify-content: space-between;
         align-items: center;
+        gap: 12px;
+        /* On a narrow viewport the thumbnail + name + button no longer fit on
+           one line; wrapping drops the Add button below instead of squeezing
+           the name under it. */
+        flex-wrap: wrap;
+    }
+
+    /* Left group: thumbnail + name/size. min-width:0 lets long names ellipsise
+       instead of pushing the Add button out of the card. */
+    .avail-main {
+        display: flex;
+        align-items: center;
+        gap: 12px;
+        min-width: 0;
+    }
+    .avail-thumb {
+        width: 52px;
+        height: 52px;
+        flex: 0 0 auto;
+        border-radius: 6px;
+        overflow: hidden;
+        background: #e9ecef;
+        border: 1px solid #dee2e6;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        font-size: 22px;
+        line-height: 1;
+    }
+    .avail-thumb img {
+        width: 100%;
+        height: 100%;
+        object-fit: cover;
+        display: block;
+    }
+    .avail-info {
+        min-width: 0;
+    }
+    .avail-name {
+        font-size: 14px;
+        /* Long media filenames carry the meaningful part at the END
+           (…_slide_003.png), so breaking mid-word is better than ellipsis,
+           which would hide the distinguishing characters. `anywhere` allows
+           breaks inside the unbroken run and keeps min-content width tiny. */
+        overflow-wrap: anywhere;
+        word-break: break-word;
+    }
+    .avail-meta {
+        font-size: 12px;
+        color: #999;
     }
     
     /* Audio toggle styles */
@@ -216,6 +270,15 @@
         color: #e2e8f0;
     }
 
+    body.dark-mode .avail-thumb {
+        background: #2d3748;
+        border-color: #4a5568;
+    }
+
+    body.dark-mode .avail-meta {
+        color: #a0aec0;
+    }
+    
     /* Dark mode for duration spinner */
     body.dark-mode .duration-display {
         background: #2d3748;
@@ -428,21 +491,32 @@
             <div class="available-content">
                 {% for content in available_content %}
                 <div class="content-item">
-                    <div>
+                    <div class="avail-main">
-                        <div>
+                        <div class="avail-thumb">
-                            {% if content.content_type == 'image' %}📷
+                            {% if content.content_type == 'image' %}
-                            {% elif content.content_type == 'video' %}🎥
+                                {# Preview the real image so the user can see what
-                            {% elif content.content_type == 'pdf' %}📄
+                                   is being added. Falls back to the camera icon
-                            {% else %}📁{% endif %}
+                                   if the file is missing or fails to load. #}
+                                <img src="{{ url_for('static', filename='uploads/' + content.current_media_path) }}"
+                                     alt="{{ content.original_display_name }}"
+                                     loading="lazy"
+                                     onerror="this.parentElement.innerHTML='<span>📷</span>';">
+                            {% elif content.content_type == 'video' %}<span>🎥</span>
+                            {% elif content.content_type == 'pdf' %}<span>📄</span>
+                            {% else %}<span>📁</span>{% endif %}
+                        </div>
+                        <div class="avail-info">
+                            <div class="avail-name" title="{{ content.original_display_name }}">
                                 {{ content.original_display_name }}
                             </div>
-                        <div style="font-size: 12px; color: #999;">
+                            <div class="avail-meta">
                                 {{ content.file_size_mb }} MB
                             </div>
                         </div>
+                    </div>
                     <form method="POST" 
                           action="{{ url_for('content.add_content_to_playlist', playlist_id=playlist.id) }}"
-                          style="display: inline;">
+                          style="display: inline; flex: 0 0 auto;">
                         <input type="hidden" name="content_id" value="{{ content.id }}">
                         <input type="hidden" name="duration" value="{{ content.duration }}">
                         <button type="submit" class="btn btn-success btn-sm">