feat(common): generic Promise-based ConfirmDialog primitive (#640 part C)

Ports 8digit/picpeak@88bfde1 — replaces `window.confirm()` with a styled,
themed, accessible in-app modal. Usage:

    const confirm = useConfirm();
    const ok = await confirm({
      title: 'Delete event?',
      message: 'This will permanently remove the gallery and all photos.',
      variant: 'danger',
      confirmLabel: 'Delete',
    });
    if (ok) doDelete();

Three variants: 'primary' (default, no icon), 'danger' (red AlertCircle +
red confirm button), 'warning' (amber AlertTriangle). Keyboard support:
Escape cancels, Enter confirms (unless focus is in an input/textarea/select
so an open form doesn't get hijacked), backdrop click cancels. Cancel button
is focused by default — a stray Enter cannot accidentally confirm a
destructive action.

Wraps at App.tsx level, inside GlobalThemeProvider so the modal respects
the theme tokens, above the toast container so a confirm appearing under a
toast still gets the click. Provider exports through components/common
alongside the rest of the shared primitives.

This PR only lands the primitive. Existing window.confirm() call-sites are
left untouched — sweeping them is follow-up work that can land in any
cadence (each sweep is one component, no architectural risk). Existing
structured-input flows (PublishGalleryDialog, DuplicateEventDialog,
PasswordResetModal, etc.) stay as-is — they collect data, not yes/no.

No new i18n entries — uses common.cancel / common.confirm / common.close
which already exist in EN + DE.

### Test plan

- [x] tsc --noEmit clean
- [x] eslint clean on changed files
- [ ] Manual: pick any existing window.confirm() site (e.g. EventDetailsPage
      delete button), swap to useConfirm(), verify the modal renders with
      theme tokens, Escape cancels, Enter confirms, backdrop click cancels,
      focus lands on Cancel
- [ ] Manual: variant='danger' renders red confirm button + AlertCircle icon
- [ ] Manual: open the dialog from inside another modal (e.g. a settings
      panel) — z-[9999] keeps the confirm on top of any other overlay

frontend/src/App.tsx

@@ -73,6 +73,7 @@ import { RequireFeature } from './components/admin/RequireFeature';
 import { PageErrorBoundary, OfflineIndicator, SkipLink, DynamicFavicon, RobotsMetaTags, CMSContentBlock, Loading } from './components/common';
 import { MaintenanceWrapper } from './components/MaintenanceWrapper';
 import { GlobalThemeProvider } from './components/GlobalThemeProvider';
+import { ConfirmDialogProvider } from './components/common';
 import { usePublicSettings } from './hooks/usePublicSettings';
 
 // Create a client
@@ -149,6 +150,7 @@ function App() {
         <MaintenanceProvider>
           <ThemeProvider>
             <GlobalThemeProvider>
+              <ConfirmDialogProvider>
               <DynamicFavicon />
               <RobotsMetaTags />
               <Router>
@@ -406,6 +408,7 @@ function App() {
               pauseOnHover
               theme={toastTheme}
             />
+              </ConfirmDialogProvider>
             </GlobalThemeProvider>
           </ThemeProvider>
         </MaintenanceProvider>