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/pages/ProceduralEditorPage.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState, useCallback } from 'react'
 import { useParams, useNavigate, useSearchParams } from 'react-router-dom'
 import { Save, ArrowLeft, ListOrdered, Wrench, Settings, FileText, Calendar, Sparkles, Layers } from 'lucide-react'
+import { Button } from '@/components/ui/Button'
 import { treesApi } from '@/api/trees'
 import { useProceduralEditorStore } from '@/store/proceduralEditorStore'
 import { CollapsibleEditorSection } from '@/components/procedural-editor/CollapsibleEditorSection'
@@ -220,21 +221,20 @@ export function ProceduralEditorPage() {
             <Sparkles className="h-4 w-4" />
             AI Assist
           </button>
-          <button
+          <Button
+            variant="secondary"
             onClick={() => handleSave('draft')}
             disabled={isSaving}
-            className="flex items-center gap-1.5 rounded-md border border-border px-3 py-2 text-sm text-muted-foreground hover:bg-accent hover:text-foreground disabled:opacity-50"
           >
             Save Draft
-          </button>
-          <button
+          </Button>
+          <Button
             onClick={() => handleSave('published')}
-            disabled={isSaving}
-            className="flex items-center gap-1.5 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"
+            loading={isSaving}
           >
             <Save className="h-4 w-4" />
             {isSaving ? 'Saving...' : 'Publish'}
-          </button>
+          </Button>
         </div>
       </div>