refactor: adopt shared Button component in 18 modal components

Replace raw <button> elements with <Button> from ui/Button.tsx:
- Primary buttons (bg-gradient-brand) → <Button variant="primary">
- Secondary buttons (border-border) → <Button variant="secondary">
- Ghost buttons → <Button variant="ghost">
- Loading states use loading prop instead of manual Loader2 spinner

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/session/ShareSessionModal.tsx

@@ -7,6 +7,7 @@ import { cn } from '@/lib/utils'
 import { toast } from '@/lib/toast'
 import { Spinner } from '@/components/common/Spinner'
 import { Modal } from '@/components/common/Modal'
+import { Button } from '@/components/ui/Button'
 
 interface ShareSessionModalProps {
   sessionId: string
@@ -180,15 +181,9 @@ export function ShareSessionModal({ sessionId, sessionLabel, isOpen, onClose }:
       size="lg"
       footer={
         <div className="flex justify-end">
-          <button
-            onClick={onClose}
-            className={cn(
-              'rounded-md border border-border px-4 py-2 text-sm font-medium text-muted-foreground',
-              'hover:bg-accent hover:text-foreground'
-            )}
-          >
+          <Button variant="secondary" onClick={onClose}>
             Close
-          </button>
+          </Button>
         </div>
       }
     >
@@ -300,17 +295,15 @@ export function ShareSessionModal({ sessionId, sessionLabel, isOpen, onClose }:
           </div>
 
           {/* Generate Button */}
-          <button
+          <Button
             onClick={handleGenerateLink}
-            disabled={isGenerating || (expirationPreset === 'custom' && !customDatetime)}
-            className={cn(
-              'flex w-full items-center justify-center gap-2 rounded-md bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20',
-              'hover:opacity-90 disabled:opacity-50 disabled:cursor-not-allowed'
-            )}
+            disabled={expirationPreset === 'custom' && !customDatetime}
+            loading={isGenerating}
+            className="w-full"
           >
             <Link2 className="h-4 w-4" />
-            {isGenerating ? 'Generating...' : 'Generate Link'}
-          </button>
+            Generate Link
+          </Button>
         </div>
 
         {/* Existing Shares */}