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

Container rendering: section (collapsible), group (bordered), tab
(multi-tab with child containers, no title header).
Cross-container tile moves via Move to Section dropdown — shows
sections, groups, and individual tabs as targets.
Container reorder via drag handles on headers.
Delete confirmation dialog with tile count.
Empty container placeholders with clickable add-tile.
Select-and-group: Shift+click tiles, Cmd+G to group, Escape to clear.

Author: alex-fedotyev