feat: replace puzzle icon with extensions-blocks icon for manager button (#8644)

## Summary

Replace the manager button puzzle icon with a custom extensions-blocks
SVG icon and add a "Manage Extensions" text label to the top bar button.

## Changes

- **What**: Swap `icon-[lucide--puzzle]` →
`icon-[comfy--extensions-blocks]` in TopMenuSection, ComfyMenuButton,
and ManagerDialog. Add visible "Manage Extensions" label (hidden below
md). Align tooltip with new label text.

## Review Focus

- Visual appearance of the new icon at different sizes
- Button layout with text label at md+ breakpoints
- Red dot notification positioning with wider button

Fixes COM-12161

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8644-feat-replace-puzzle-icon-with-extensions-blocks-icon-for-manager-button-2fe6d73d3650815c8867efc5a0842ef7)
by [Unito](https://www.unito.io)

Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/components/sidebar/ComfyMenuButton.vue

@@ -230,7 +230,7 @@ const extraMenuItems = computed(() => [
   {
     key: 'manage-extensions',
     label: t('menu.manageExtensions'),
-    icon: 'icon-[lucide--puzzle]',
+    icon: 'icon-[comfy--extensions-blocks]',
     command: showManageExtensions
   }
 ])