Add integrated downloads manager and UI

Implements a full downloads manager with Electron main process handling, IPC APIs, and renderer integration. Adds a dedicated downloads page, a mini downloads popup in the navigation bar with progress ring, and controls for pausing, resuming, canceling, opening, and showing downloads. Updates styles and navigation to support the new downloads features.

renderer/index.html

@@ -40,7 +40,23 @@
     </div>
 
     <div class="nav-right">
-      <button title="Downloads">⭳</button>
+      <div class="downloads-wrapper">
+        <button id="downloads-btn" title="Downloads" aria-label="Downloads">
+          <svg width="18" height="18" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">
+            <path d="M12 3v12"/>
+            <path d="M7 10l5 5 5-5"/>
+            <path d="M5 21h14"/>
+          </svg>
+        </button>
+        <div id="downloads-popup" class="hidden">
+          <div class="downloads-pop-header">
+            <span>Downloads</span>
+            <button id="downloads-show-all">Show all</button>
+          </div>
+          <div id="downloads-list" class="downloads-pop-list"></div>
+          <div id="downloads-empty" class="downloads-empty">No downloads</div>
+        </div>
+      </div>
       <div class="menu-wrapper">
         <button id="menu-btn">☰</button>
         <div id="menu-popup" class="hidden">