feat: Integrate containers, tabs, and polish

Fill-right-then-wrap tile positioning via calculateNextTilePosition
(w=8, h=10 matching original defaults). Floating drag bar on hover
(no content shift). Move dropdown: tab targets shown as pipe-separated
siblings with target in normal weight, others dimmed (Logs | Metrics).
Section empty state wired with onAddTile. Cross-container moves via
dropdown. Delete confirmation always shown. Select-and-group Cmd+G.
makeId uses larger ID space to reduce collision probability.

Author: alex-fedotyev