fix(mosaic): gate animations behind animationReady, remove loading=lazy

- Animation classes only applied after height measurement completes
- Removed loading="lazy" so images load eagerly for accurate scrollHeight

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/Mosaic.svelte

@@ -160,8 +160,8 @@
     {#each columns as colStories, colIdx}
       <div
         class="mosaic-column"
-        class:scroll-up={colIdx % 2 === 0}
-        class:scroll-down={colIdx % 2 === 1}
+        class:scroll-up={animationReady && colIdx % 2 === 0}
+        class:scroll-down={animationReady && colIdx % 2 === 1}
         class:paused={pausedColumn === colIdx}
         style="--scroll-duration: {columnDurations[colIdx] || 30}s;"
       >
@@ -172,7 +172,7 @@
             onmouseenter={(e) => handleMouseEnter(story, colIdx, e)}
             onmouseleave={handleMouseLeave}
           >
-            <img src={story.poster} alt={story.name} loading="lazy" />
+            <img src={story.poster} alt={story.name} />
           </div>
         {/each}
         <!-- Duplicate for seamless loop -->
@@ -182,7 +182,7 @@
             onmouseenter={(e) => handleMouseEnter(story, colIdx, e)}
             onmouseleave={handleMouseLeave}
           >
-            <img src={story.poster} alt={story.name} loading="lazy" />
+            <img src={story.poster} alt={story.name} />
           </div>
         {/each}
       </div>