docs: add comprehensive Stripe billing guide

Covers architecture, configuration steps, user flows, API routes,
webhooks, pricing, testing with Stripe CLI, production checklist,
and troubleshooting.

memento-note/components/note-card.tsx

@@ -351,9 +351,9 @@ export const NoteCard = memo(function NoteCard({
       refreshNotes(note?.notebookId)
 
       if (!note.isPinned) {
-        toast.success(t('notes.pinned') || 'Note pinned')
+        toast.success(t('notes.pinned'))
       } else {
-        toast.info(t('notes.unpinned') || 'Note unpinned')
+        toast.info(t('notes.unpinned'))
       }
     })
   }
@@ -483,8 +483,8 @@ export const NoteCard = memo(function NoteCard({
       {/* Drag Handle - Only visible on mobile/touch devices */}
       <div
         className="muuri-drag-handle absolute top-2 start-2 z-20 cursor-grab active:cursor-grabbing p-2 md:hidden"
-        aria-label={t('notes.dragToReorder') || 'Drag to reorder'}
-        title={t('notes.dragToReorder') || 'Drag to reorder'}
+        aria-label={t('notes.dragToReorder')}
+        title={t('notes.dragToReorder')}
       >
         <GripVertical className="h-5 w-5 text-muted-foreground" />
       </div>
@@ -563,7 +563,7 @@ export const NoteCard = memo(function NoteCard({
              <DropdownMenuSeparator />
              <DropdownMenuItem className="text-muted-foreground" onSelect={() => onCreateSubNotebook?.()}>
                <Plus className="h-4 w-4 me-2" />
-               {t('notebook.createSubNotebook') || 'Nouveau sous-carnet…'}
+               {t('notebook.createSubNotebook')}
              </DropdownMenuItem>
           </DropdownMenuContent>
         </DropdownMenu>
@@ -605,7 +605,7 @@ export const NoteCard = memo(function NoteCard({
           <button
             onClick={handleRemoveFusedBadge}
             className="ms-1 opacity-0 group-hover/badge:opacity-100 hover:opacity-100 transition-opacity"
-            title={t('notes.remove') || 'Remove'}
+            title={t('notes.remove')}
           >
             <Trash2 className="h-2.5 w-2.5" />
           </button>
@@ -873,13 +873,13 @@ export const NoteCard = memo(function NoteCard({
           <AlertDialogHeader>
             <AlertDialogTitle>{t('notes.confirmDeleteTitle') || t('notes.delete')}</AlertDialogTitle>
             <AlertDialogDescription>
-              {t('notes.confirmDelete') || 'Are you sure you want to delete this note?'}
+              {t('notes.confirmDelete')}
             </AlertDialogDescription>
           </AlertDialogHeader>
           <AlertDialogFooter>
-            <AlertDialogCancel>{t('common.cancel') || 'Cancel'}</AlertDialogCancel>
+            <AlertDialogCancel>{t('common.cancel')}</AlertDialogCancel>
             <AlertDialogAction variant="destructive" onClick={handleDelete}>
-              {t('notes.delete') || 'Delete'}
+              {t('notes.delete')}
             </AlertDialogAction>
           </AlertDialogFooter>
         </AlertDialogContent>