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 */}

web/src/app/home/knowledge/KBDetailContent.tsx

@@ -146,7 +146,12 @@ export default function KBDetailContent({ id }: { id: string }) {
           <h1 className="text-xl font-semibold">
             {t('knowledge.editKnowledgeBase')}
           </h1>
-          <Button type="submit" form="kb-form" disabled={!formDirty}>
+          <Button
+            type="submit"
+            form="kb-form"
+            disabled={!formDirty}
+            className={activeTab !== 'metadata' ? 'invisible' : ''}
+          >
             {t('common.save')}
           </Button>
         </div>

web/src/app/home/pipelines/PipelineDetailContent.tsx

@@ -80,7 +80,12 @@ export default function PipelineDetailContent({ id }: { id: string }) {
       {/* Sticky Header: title + save button */}
       <div className="flex items-center justify-between pb-4 shrink-0">
         <h1 className="text-xl font-semibold">{t('pipelines.editPipeline')}</h1>
-        <Button type="submit" form="pipeline-form" disabled={!formDirty}>
+        <Button
+          type="submit"
+          form="pipeline-form"
+          disabled={!formDirty}
+          className={activeTab !== 'config' ? 'invisible' : ''}
+        >
           {t('common.save')}
         </Button>
       </div>