fix: compose button moved to top-right of window (not sidebar)

The compose button was inside the left sidebar (position: absolute relative
to .left-rail). Moved it outside the sidebar in the HTML and changed CSS
from position: absolute to position: fixed so it sits at the top-right
corner of the entire window.

Verified with vision model (Qwen3.6-27B):
"The compose button is located in the top-right corner of the entire window,
just outside the main content area. It is not inside the sidebar."

This is the first fix made using the visual eyes system — the agent could
verify the change before Nick tests it.

frontend/src/app-base/AppBaseShell.svelte

@@ -135,6 +135,20 @@
   </button>
 
   <BreakSuggestion quietMode={settingsStore.quietMode} />
+
+  <!-- Compose button — top-right of window (not inside sidebar) -->
+  <button
+    type="button"
+    class="nav-compose-icon"
+    use:tooltip={{ content: 'Open Compose', placement: 'left' }}
+    aria-label="Open Compose"
+    aria-expanded={isComposeOpen}
+    aria-controls="compose-panel"
+    onclick={toggleCompose}
+  >
+    <span aria-hidden="true">♫</span>
+  </button>
+
   <aside class="left-rail" aria-label="Primary navigation">
     <div class="brand-lockup">
       <span class="brand-mark" aria-label="Conductor C app mark">{activeRoute.brandMark}</span>
@@ -150,18 +164,6 @@
       onToggleCompose={toggleCompose}
     />
 
-    <button
-      type="button"
-      class="nav-compose-icon"
-      use:tooltip={{ content: 'Open Compose', placement: 'right' }}
-      aria-label="Open Compose"
-      aria-expanded={isComposeOpen}
-      aria-controls="compose-panel"
-      onclick={toggleCompose}
-    >
-      <span aria-hidden="true">♫</span>
-    </button>
-
     <nav aria-label="Primary sections">
       {#each appBaseNavigationItems as item (item.routeId)}
         <button

frontend/src/styles/app-base-shell.scss

@@ -193,7 +193,7 @@ p {
 }
 
 .nav-compose-icon {
-  position: absolute;
+  position: fixed;
   top: 1.25rem;
   right: 1.25rem;
   display: inline-grid;
@@ -207,11 +207,7 @@ p {
   background: rgb(139 156 255 / 12%);
   color: #f5f7fb;
   cursor: pointer;
-  z-index: 10;
-}
-
-.left-rail {
-  position: relative;
+  z-index: 100;
 }
 
 .nav-compose-icon:hover,