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/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>
       )}