feat: F025 button & action logic audit — global toast, confirm dialog, wired mutations

- Add ToastProvider (global MUI Snackbar stack) + toastStore (Zustand)
- Add ConfirmDialog reusable component for destructive actions
- Wire toast feedback to all async mutations: review status, AI output status,
  match approval, shortlist add/remove/title/finalize, signal review actions
- Fix dead UserMenu buttons (Profil, Einstellungen) — show info toast
- Migrate local Snackbar instances in AddToShortlistDialog and
  FutureSignalDetailPanel to global toast store

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

src/components/layout/AppShell.tsx

@@ -41,6 +41,7 @@ import { RightContextPanel } from './RightContextPanel'
 import { CompareTray } from './CompareTray'
 import { GlobalAIAssistantDrawer, GlobalAIAssistantButton } from '../assistant'
 import { useAssistantStore } from '../../stores/assistantStore'
+import { ToastProvider } from '../ui'
 
 // ---------------------------------------------------------------------------
 // Types
@@ -546,6 +547,7 @@ export function AppShell() {
       <CompareTray />
       <GlobalAIAssistantDrawer />
       <GlobalAIAssistantButton />
+      <ToastProvider />
     </Box>
   )
 }