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/step-library/StepDetailModal.tsx

@@ -4,6 +4,7 @@ import { cn } from '@/lib/utils'
 import { MarkdownContent } from '@/components/ui/MarkdownContent'
 import { stepsApi } from '@/api/steps'
 import type { Step, Review } from '@/types/step'
+import { Button } from '@/components/ui/Button'
 
 interface StepDetailModalProps {
   stepId: string
@@ -318,22 +319,20 @@ export function StepDetailModal({ stepId, onClose, onInsert }: StepDetailModalPr
 
         {/* Footer - Actions */}
         <div className="flex gap-2 border-t border-border p-4">
-          <button
+          <Button
+            variant="secondary"
             onClick={onClose}
-            className="flex-1 rounded-md border border-border px-4 py-2 text-sm font-medium text-muted-foreground hover:bg-accent hover:text-foreground"
+            className="flex-1"
           >
             Cancel
-          </button>
-          <button
+          </Button>
+          <Button
             onClick={handleInsert}
             disabled={!step}
-            className={cn(
-              'flex-1 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'
-            )}
+            className="flex-1"
           >
             Insert Into Session
-          </button>
+          </Button>
         </div>
       </div>
     </div>

frontend/src/components/step-library/StepForm.tsx

@@ -3,6 +3,7 @@ import { Plus, X, HelpCircle, Zap, CheckCircle } from 'lucide-react'
 import { cn } from '@/lib/utils'
 import { stepCategoriesApi } from '@/api/stepCategories'
 import type { StepCreate, StepCategory, StepCommand } from '@/types/step'
+import { Button } from '@/components/ui/Button'
 
 interface StepFormProps {
   onSubmit: (data: StepCreate) => void
@@ -369,20 +370,21 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
 
       {/* Actions */}
       <div className="flex gap-2 pt-4">
-        <button
+        <Button
           type="button"
+          variant="secondary"
           onClick={onCancel}
-          className="flex-1 rounded-md border border-border px-4 py-2 text-sm font-medium text-muted-foreground hover:bg-accent hover:text-foreground"
+          className="flex-1"
         >
           Cancel
-        </button>
-        <button
+        </Button>
+        <Button
           type="submit"
-          disabled={isSubmitting}
-          className="flex-1 rounded-md bg-gradient-brand text-white shadow-lg shadow-primary/20 px-4 py-2 text-sm font-medium hover:opacity-90 disabled:opacity-50"
+          loading={isSubmitting}
+          className="flex-1"
         >
-          {isSubmitting ? 'Saving...' : (submitLabel ?? 'Insert Step')}
-        </button>
+          {submitLabel ?? 'Insert Step'}
+        </Button>
       </div>
     </form>
   )

frontend/src/components/step-library/StepLibraryBrowser.tsx

@@ -1,5 +1,6 @@
 import { useState, useEffect, useMemo } from 'react'
 import { Search, ChevronDown, ChevronUp, Loader2 } from 'lucide-react'
+import { Button } from '@/components/ui/Button'
 import { cn } from '@/lib/utils'
 import { stepsApi } from '@/api/steps'
 import { stepCategoriesApi } from '@/api/stepCategories'
@@ -253,12 +254,9 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
         ) : error ? (
           <div className="rounded-lg border border-red-400/20 bg-red-400/10 p-4 text-center">
             <p className="text-sm text-red-400 mb-3">{error}</p>
-            <button
-              onClick={() => setRetryCount(c => c + 1)}
-              className="rounded-md border border-border px-3 py-1.5 text-sm text-muted-foreground hover:bg-accent hover:text-foreground transition-colors"
-            >
+            <Button variant="secondary" size="sm" onClick={() => setRetryCount(c => c + 1)}>
               Try again
-            </button>
+            </Button>
           </div>
         ) : steps.length === 0 ? (
           <div className="rounded-lg border border-border bg-accent/50 p-12 text-center">
@@ -374,12 +372,9 @@ export function StepLibraryBrowser({ onInsert, onCreateNew, showCreateButton = f
       {/* Footer - Optional Create Button */}
       {showCreateButton && onCreateNew && (
         <div className="border-t border-border p-4">
-          <button
-            onClick={onCreateNew}
-            className="w-full rounded-md bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
-          >
+          <Button onClick={onCreateNew} className="w-full">
             + Create New Step
-          </button>
+          </Button>
         </div>
       )}