feat(applications): standalone application shell with its own menu

[rubenvdlinde]

Rebuild of #25 on application pages (nextcloud#3075, see #13 and the reply on nextcloud#3075). Stacked on stack-4-main (page stack #35-#38 with the menu stack #20-#23 merged in).

What changes

An application opened on its own, at /apps/tables/app/{id}, gets its own navigation: its menu replaces the Tables navigation, and every menu item opens inside the application.

• ApplicationNavigation: the application's icon, name and description, then its menu, nested one level like the stored menu. A link opens outside the app. The owner gets "Configure in Tables".
• Menu item route /application/{id}/menu/{technicalName}: a page item opens that page of the application on the grid (GridPage, the dashboard item opens the start page), a view or table item shows its rows (DataWidget, the same component as a data widget), an unknown or broken item shows an empty state.
• First page: opened on its own, the application starts on its first menu item that is not a link. For a new application that is the dashboard item.
• Inside Tables: a menu item route shows a breadcrumb back to the application and an "Open application" button.
• MenuItemIcon: a warning icon for a broken item, else the chosen icon, else one for what the item opens.
• Focus: the content area only takes focus after a first click or key press, so no focus ring is drawn on load.

Carried over from #25, and what changed because of pages

• Carried over as is: App.vue (shell switch, focus), ApplicationNavigation, MenuItemIcon, menuItemRoute/menuTree, the route, the breadcrumb.
• The menu target is no longer loaded as a view (grid views are gone): page targets render the application page from the page API, view/table targets reuse DataWidget instead of a copy of the old start page loading code in Context.vue.
• feat(applications): standalone application shell with its own menu #25 showed an "Overview" entry with the application's tables and views. Those now live on the start page (the dashboard), so the entry opens the start page, and only shows when no menu item opens it (applications created before the menu existed). "Grid views as resources" is dropped: there are no grid views.
• Opened on its own, the application header and page bar of feat(pages): show every application page on the grid #38 are hidden: the menu is the navigation.

Tests

Nextcloud 34 container (PostgreSQL 16), Playwright at 1600 x 1000:

• playwright/e2e/application.spec.ts (new): opened on its own, the menu shows 4 items, starts on /menu/dashboard with the start page's table, a page item opens its grid page, a table item its rows, a link keeps its URL, no focus on load; inside Tables the breadcrumb and "Open application"; a page item whose page was deleted shows the warning icon and the empty state; without a dashboard item the start page gets its own entry. 4 passed.
• context-grid-pages.spec.ts, context-menu.spec.ts, context-navigation.spec.ts, context.spec.ts: 13 passed (run with --timeout=120000, the CI value; the local instance is slow).
• npx cypress run --component --spec cypress/component/menuItems.cy.js: 14 passing (page, view and table routes, menu tree).
• npm run lint: 0 errors (1 inherited warning in store.js), npm run stylelint clean, npm run build OK.

Inherited: the create endpoint answers with pages[].pageType (entity serialisation) while the read endpoint uses page_type; the test reads the start page id from the dashboard menu item instead.

Prepared in the Conduction fork for porting upstream by @remko48. Prepared with Claude Code.