Make every session tab speak the same drag language as a sidebar row, and
keep tab focus 1:1 across the sidebar, tiles, and main:
- Drag a session tile's OR the main workspace tab onto a composer to link the
chat (@session chip), or onto a zone/edge to stack/split — tabDrag now
returns whether it took the drag, so the workspace tab defers to the generic
pane move on a fresh draft (nothing to link).
- A lone tool panel (terminal/logs) dragged to its own zone keeps its header,
so it stays draggable/closable instead of stranding a dead, tab-less zone.
- The sidebar highlight follows the FOCUSED session (interacted tile, else the
main selection) rather than only the main one.
- Clicking an already-open session jumps to its tab — an open tile, or the
workspace tab when it's the main session and focus sits on a tile — instead
of a dead no-op reload.
- Secondary (single-chat pop-out) windows boot to the default tree with no
tiles and never persist their stripped-down layout back, so they can't
inherit or clobber the primary window's tabs/splits.
The pointer drag ghost is extracted to a shared lib/drag-ghost and the drop
affordances drop their now-unused labels.
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