fix(kon): task sidebar overlay, Parakeet model management, conditional engine settings

TaskSidebar:
- Changed from flex layout member to fixed overlay with backdrop
- Click backdrop or press Escape to close
- Content no longer shrinks when sidebar opens — buttons stay accessible
- Positioned at z-50 over main content, top offset for titlebar

Settings — Parakeet model management:
- Added Parakeet model state: download, check, load status
- New downloadParakeet() and loadParakeet() functions
- Listens for parakeet-download-progress events
- Parakeet section shows download/load/status controls

Settings — conditional engine sections:
- Whisper model selector only shows when engine === "whisper"
- Parakeet model controls only show when engine === "parakeet"
- Cleaner UI — only active engine's options visible

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/routes/+layout.svelte

@@ -98,7 +98,7 @@
   });
 </script>
 
-<svelte:window onmousemove={handleMouseMove} />
+<svelte:window onmousemove={handleMouseMove} onkeydown={(e) => { if (e.key === "Escape" && page.taskSidebarOpen) page.taskSidebarOpen = false; }} />
 
 {#if page.recording}
   <div
@@ -113,16 +113,27 @@
 {:else}
   <div class="flex flex-col h-screen w-screen overflow-hidden grain">
     <Titlebar />
-    <div class="flex flex-1 min-h-0">
+    <div class="flex flex-1 min-h-0 relative">
       {#if page.current !== "first-run"}
         <Sidebar />
       {/if}
       <div class="flex-1 overflow-hidden bg-bg">
         {@render children()}
       </div>
-      {#if page.taskSidebarOpen && page.current !== "first-run"}
-        <TaskSidebar />
-      {/if}
     </div>
   </div>
+
+  <!-- Task sidebar as overlay — doesn't shrink main content -->
+  {#if page.taskSidebarOpen && page.current !== "first-run" && !isSecondaryWindow}
+    <!-- Backdrop -->
+    <button
+      class="fixed inset-0 bg-black/20 z-40"
+      onclick={() => page.taskSidebarOpen = false}
+      aria-label="Close task sidebar"
+    ></button>
+    <!-- Sidebar panel -->
+    <div class="fixed top-[36px] right-0 bottom-0 z-50 animate-slide-in-right">
+      <TaskSidebar />
+    </div>
+  {/if}
 {/if}