feat: Integrate containers, tabs, and polish into dashboard page

Tile drag bar restored at top of each tile for within-grid drag.
Cross-container moves via Move to Section dropdown, which shows
tab entries as 'Container > Tab Name' format.

Container rendering: section (collapsible), group (bordered), tab
(tabbed view filtering tiles by containerId + tabId).

Delete confirmation dialog with tile count.
Empty container placeholders with clickable add-tile.
Select-and-group: Shift+click tiles then Cmd+G.

Author: alex-fedotyev