fix(home): separate mega-cta from tile-link-overlay (was overlapping with objective)

Before: .tile-link-overlay contained the .mega-cta text. Both overlay and .mega-objective were positioned at the bottom of the tile, causing visual overlap.
After: .mega-cta is a normal-flow <span> in tile-body (pushed to bottom via margin-top: auto). .tile-link-overlay is now an empty click target (no inner content, no padding, no flex).

src/pages/index.astro

@@ -210,9 +210,8 @@ const featuredPortfolio = portfolio.filter(p =>
                 <span class="objective-label">เป้าหมาย:</span>
                 <span class="objective-value">{copy.objective}</span>
               </div>
-              <a href={`/services/${s.id}`} class="tile-link-overlay">
-                <span class="mega-cta">ดูรายละเอียด →</span>
-              </a>
+              <span class="mega-cta">ดูรายละเอียด →</span>
+              <a href={`/services/${s.id}`} class="tile-link-overlay" aria-label={`ดูรายละเอียด ${s.data.title}`}></a>
             </BentoTile>
           );
         })}