fix(web): prevent tab list layout shift when save button toggles visibility

Use invisible class instead of conditional rendering for save buttons
in bot, pipeline, and knowledge base detail pages, so the button always
occupies space and the tab list position stays stable across tab switches.

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

web/src/app/home/bots/BotDetailContent.tsx

@@ -174,11 +174,14 @@ export default function BotDetailContent({ id }: { id: string }) {
               </div>
             )}
           </div>
-          {activeTab === 'config' && (
-            <Button type="submit" form="bot-form" disabled={!formDirty}>
-              {t('common.save')}
-            </Button>
-          )}
+          <Button
+            type="submit"
+            form="bot-form"
+            disabled={!formDirty}
+            className={activeTab !== 'config' ? 'invisible' : ''}
+          >
+            {t('common.save')}
+          </Button>
         </div>
 
         {/* Horizontal Tabs */}