refactor: enforce shared Modal component (#100)

* refactor: enforce shared Modal component in remaining custom modals

- ShareSessionModal: replaced custom modal markup with <Modal>
- CreateCategoryModal: replaced custom modal markup with <Modal>
- EditCategoryModal: replaced custom modal markup with <Modal>
- All now get focus trapping, Escape close, body scroll lock for free

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

* 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>

* refactor: adopt shared Button component in 20 page/component files

Replace raw <button> elements with <Button> across pages and remaining
components. 38 total files now use the shared Button component consistently.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/session/ForkTreeModal.tsx

@@ -1,7 +1,8 @@
 import { useState } from 'react'
 import { Modal } from '@/components/common/Modal'
-import { GitFork, Loader2 } from 'lucide-react'
+import { GitFork } from 'lucide-react'
 import { cn } from '@/lib/utils'
+import { Button } from '@/components/ui/Button'
 
 interface ForkTreeModalProps {
   isOpen: boolean
@@ -44,38 +45,21 @@ export function ForkTreeModal({
 
   const footer = (
     <div className="flex justify-end gap-3">
-      <button
+      <Button
+        variant="ghost"
         onClick={onSkip}
         disabled={isSaving}
-        className={cn(
-          'rounded-md px-4 py-2 text-sm font-medium transition-colors',
-          'text-muted-foreground hover:bg-accent hover:text-foreground',
-          'disabled:cursor-not-allowed disabled:opacity-50'
-        )}
       >
         Skip
-      </button>
-      <button
+      </Button>
+      <Button
         onClick={handleFork}
-        disabled={isSaving || !name.trim()}
-        className={cn(
-          'flex items-center gap-2 rounded-md bg-gradient-brand text-white shadow-lg shadow-primary/20 px-4 py-2 text-sm font-medium transition-colors',
-          'hover:opacity-90',
-          'disabled:cursor-not-allowed disabled:opacity-50'
-        )}
+        disabled={!name.trim()}
+        loading={isSaving}
       >
-        {isSaving ? (
-          <>
-            <Loader2 className="h-4 w-4 animate-spin" />
-            Saving...
-          </>
-        ) : (
-          <>
-            <GitFork className="h-4 w-4" />
-            Save as Personal Tree
-          </>
-        )}
-      </button>
+        <GitFork className="h-4 w-4" />
+        Save as Personal Tree
+      </Button>
     </div>
   )