Fix click-dead zone on the Assistant launcher tab

The document root kept a forced scrollbar (html { overflow-y: scroll })
from before the app-scroll-shell owned scrolling. The root never
scrolls, so the rule only reserved a dead 11px gutter at the right
viewport edge and pulled every fixed right-anchored element off the
true screen edge - parking the desktop Assistant launcher tab under
the scroll shell's own scrollbar, which hit-tests above descendants.
Net effect: the tab's right quarter plus the whole screen-edge strip
silently swallowed clicks, exactly where an edge-docked control gets
aimed at.

Drop the legacy root-scrollbar rule and portal the launcher out of the
scroll shell so its full face receives clicks, flush to the real edge.

Verified live: elementFromPoint returns the button across its entire
width including the last pixel column, and an extreme-edge click opens
the drawer.

frontend-modern/src/AppLayout.tsx

@@ -1,4 +1,5 @@
 import { For, Show, Suspense, createEffect, createMemo, createSignal, onCleanup } from 'solid-js';
+import { Portal } from 'solid-js/web';
 import type { JSX } from 'solid-js';
 import { useLocation, useNavigate } from '@solidjs/router';
 import BellIcon from 'lucide-solid/icons/bell';
@@ -976,15 +977,20 @@ export function AppLayout(props: AppLayoutProps) {
           !dialogStackHasBlockingDialog()
         }
       >
-        <button
-          type="button"
-          onClick={() => aiChatStore.open(assistantPageContext().context)}
-          class={AI_CHAT_LAUNCHER_BUTTON_CLASS}
-          title={assistantPageContext().title}
-          aria-label={assistantPageContext().ariaLabel}
-        >
-          <SparklesIcon class="h-5 w-5 flex-shrink-0" />
-        </button>
+        {/* Portaled out of .app-scroll-shell: as a descendant the scroll
+            container's own scrollbar hit-tests above the tab's right edge,
+            leaving the screen-edge click zone dead. */}
+        <Portal>
+          <button
+            type="button"
+            onClick={() => aiChatStore.open(assistantPageContext().context)}
+            class={AI_CHAT_LAUNCHER_BUTTON_CLASS}
+            title={assistantPageContext().title}
+            aria-label={assistantPageContext().ariaLabel}
+          >
+            <SparklesIcon class="h-5 w-5 flex-shrink-0" />
+          </button>
+        </Portal>
       </Show>
     </div>
   );

frontend-modern/src/index.css

@@ -132,11 +132,11 @@
     color: var(--color-text-base);
   }
 
-  /* Hide scrollbars during transitions to prevent flicker */
-  html {
-    overflow-y: scroll;
-    scroll-behavior: smooth;
-  }
+  /* The document root never scrolls: the app shell is h-screen with
+     overflow hidden and .app-scroll-shell owns vertical scrolling.
+     Forcing a root scrollbar here would reserve a dead gutter at the
+     right viewport edge and shift every fixed right-anchored element
+     (e.g. the Assistant launcher tab) off the true screen edge. */
 }
 
 @layer components {