design: landing page effects and polish

- radial gradient glow background
- card hover: translateY(-4px) + blue glow shadow
- gradient on cards for depth
- OSCloud logo in nav instead of blue M
- wider card grid and footer (1100px)
- footer separator between OSCloud and links
- vertically centered content
- faster transitions (0.15s)

index.html

@@ -14,7 +14,7 @@
 
   <nav class="site-nav">
     <a href="/" class="nav-logo">
-      <div class="nav-logo-icon">M</div>
+      <img src="./index/img/logo_oscloud.png" alt="OSCloud" style="width: 32px; height: auto; border-radius: 6px;" />
       <span class="nav-logo-text">mxchat.cz</span>
     </a>
     <button class="nav-menu-btn" onclick="document.querySelector('.nav-links').classList.toggle('open')" aria-label="Menu">&#9776;</button>
@@ -99,6 +99,7 @@
 
   <footer class="site-footer">
     <span>&copy; 2024-2026 OSCloud</span>
+    <span style="color: var(--text-dim); margin: 0 12px;">&middot;</span>
     <div class="footer-links">
       <a href="https://mamutovo.cz/@oscloud" target="_blank" rel="noopener"><i class="fa fa-mastodon"></i> Mastodon</a>
       <a href="https://matrix.to/#/#podpora:mxchat.cz" target="_blank" rel="noopener"><i class="fa fa-matrix-org"></i> Matrix</a>