Refresh screenshot galleries and repair portal upgrades

Use the September 30 hero on the website and README, add a shared screenshot gallery, and remove the old showcase images. Build image links locally and clean generated site output.

Back up the known development portal override during installation and report custom overrides before building. Keep source selections clear while letting blur show through unselected rows.

site/index.html

@@ -19,7 +19,7 @@
     <meta property="og:type" content="website" />
     <meta
       property="og:image"
-      content="https://ferese-wm.github.io/ferese/assets/ferese-desktop.png"
+      content="https://ferese-wm.github.io/ferese/assets/screenshots/ferese-main-hero.png"
     />
     <meta name="twitter:card" content="summary_large_image" />
     <title>Ferese — Wayland desktop</title>
@@ -37,6 +37,7 @@
         >
         <nav aria-label="Main navigation">
           <a href="./" aria-current="page">Overview</a
+          ><a href="docs/screenshots.html">Screenshots</a
           ><a href="docs/">Documentation</a
           ><a href="https://github.com/ferese-wm/ferese"
             >GitHub <svg class="link-arrow" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.8" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true" focusable="false"><path d="M7 17 17 7M7 7h10v10"/></svg></a
@@ -63,14 +64,14 @@
       </section>
       <figure class="product-screenshot page-width">
         <a
-          href="assets/ferese-desktop.webp"
+          href="assets/screenshots/ferese-main-hero.png"
           aria-label="View the full-resolution Ferese desktop screenshot"
         >
           <img
-            src="assets/ferese-desktop.webp"
+            src="assets/screenshots/ferese-main-hero.png"
             width="2880"
             height="1800"
-            alt="Ferese Blue desktop with the default wallpaper, Settings, Control Center and clock widget"
+            alt="Ferese Blue desktop with Settings, Control Center and the default wallpaper"
             fetchpriority="high"
           />
         </a>