fix: orphan centering — switch inner grid to flex-wrap + justify-center

CSS grid + mx-auto centers the whole block but not orphan tiles within
their row (e.g. 5 tiles @1685x1342, the 2-tile row was left-aligned).
Switched to display:flex; flex-wrap:wrap; justify-content:center with
explicit width/height on each tile wrapper. This naturally centers every
row, including orphan rows with fewer tiles.

The computeUniformGrid algorithm is unchanged — still uniform px tile
sizes with aspect flexing in [3:4..16:9].

src/app/components/VideoCallContent.svelte

@@ -295,10 +295,12 @@
         class="min-h-0 flex-1 overflow-y-auto">
         {#if tileGrid}
           <div
-            class="mx-auto grid content-start gap-2"
-            style={`grid-template-columns: repeat(${tileGrid.cols}, ${tileGrid.tileWidth}px); grid-template-rows: repeat(${tileGrid.rows}, ${tileGrid.tileHeight}px)`}>
+            class="flex flex-wrap justify-center gap-2"
+            style={`max-width: ${tileGrid.cols * tileGrid.tileWidth + (tileGrid.cols - 1) * 8}px`}>
             {#each videoTiles as tile (tileKey(tile))}
-              <div class="overflow-hidden rounded-2xl">
+              <div
+                class="overflow-hidden rounded-2xl shrink-0"
+                style={`width: ${tileGrid.tileWidth}px; height: ${tileGrid.tileHeight}px`}>
                 {@render videoTile(tile, "default")}
               </div>
             {/each}