feat: complete i18n audit fix, toast system, UI improvements
i18n (all 3 languages - EN, ES, PT-BR):
- ApiKeys page + CreateApiKeyDialog: 26 strings
- billing/Processing page: 8 strings
- onboarding/Step1 + Step2: 10 strings (new onboarding.php)
- UserMenuContent: 7 strings (Account, Theme, Language, Log out)
- PhotoUpload: 4 strings (Upload, Remove, hint)
- TimezoneCombobox: 3 strings
- DatePicker: 1 string
- workspaces/Create + Index: full i18n (new workspaces.php)
Toast system:
- Unify Toast.vue with Sonner (copied from sendkit)
- Flash messages + JS toasts use same system
- Icon colors via CSS (success/error/warning/info)
- Position bottom-right
UI improvements:
- Posts Edit: move trash/schedule/publish to header-right slot
- Posts Edit: labels/hashtags/datepicker stay in top bar with platforms
- Posts Edit: delete post requires typing DELETE to confirm
- Posts Edit: Labels button with icon+text, always visible
- DatePicker: w-full by default, w-auto via class override
- Fix "Etiquetas" -> "Etiqueta" (remove plural)
- Posts breadcrumbs: /posts shows "Posts > All Posts"
- Calendar breadcrumbs: "Posts > Calendar"
- Edit breadcrumbs: "Posts > Edit Post"
- Remove unused AppHeader.vue and AppHeaderLayout.vue
All 728 tests passing.