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/SessionDetailPage.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useState } from 'react'
 import { useParams, useNavigate } from 'react-router-dom'
 import { Copy, Check, Eye, Save, Share2, CheckCircle2, AlertTriangle, ArrowUpRight, HelpCircle, Flag } from 'lucide-react'
+import { Button } from '@/components/ui/Button'
 import { sessionsApi } from '@/api/sessions'
 import { stepsApi } from '@/api/steps'
 import { ExportPreviewModal } from '@/components/session/ExportPreviewModal'
@@ -391,13 +392,10 @@ export function SessionDetailPage() {
               </div>
             </div>
             {/* Primary action: Copy for Ticket */}
-            <button
-              onClick={handleCopyForTicket}
-              className="flex shrink-0 items-center gap-2 rounded-lg bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
-            >
+            <Button onClick={handleCopyForTicket} className="shrink-0">
               {copiedPsa ? <Check className="h-4 w-4" /> : <Copy className="h-4 w-4" />}
               {copiedPsa ? 'Copied!' : 'Copy for Ticket'}
-            </button>
+            </Button>
           </div>
         </div>
       ) : !session.completed_at ? (
@@ -410,12 +408,9 @@ export function SessionDetailPage() {
               <p className="text-xs text-muted-foreground">Set an outcome to finalize this session and generate documentation.</p>
             </div>
           </div>
-          <button
-            onClick={() => setShowOutcomeModal(true)}
-            className="shrink-0 rounded-lg bg-gradient-brand px-4 py-2 text-sm font-medium text-white shadow-lg shadow-primary/20 hover:opacity-90"
-          >
+          <Button onClick={() => setShowOutcomeModal(true)} className="shrink-0">
             Complete Session
-          </button>
+          </Button>
         </div>
       ) : null}
 
@@ -462,14 +457,15 @@ export function SessionDetailPage() {
         >
           {copied ? <Check className="h-4 w-4 text-emerald-400" /> : <Copy className="h-4 w-4" />}
         </button>
-        <button
+        <Button
+          variant="secondary"
+          size="sm"
           onClick={handlePreview}
           disabled={isExporting}
-          className="flex items-center gap-1.5 rounded-md border border-border bg-card px-3 py-1.5 text-sm text-muted-foreground hover:bg-accent hover:text-foreground disabled:opacity-50"
         >
           <Eye className="h-4 w-4" />
           {isExporting ? 'Loading...' : 'Preview'}
-        </button>
+        </Button>
         {/* Copy for ticket (secondary position when session is complete) */}
         {session.completed_at && (
           <button