…tile

Replace the group-and-tabs layout model with a flat Split | Tile tree:
each leaf is a single tile, no tabs within a tile, and dividers / drop
targets share the sidebar's hairline + accent-on-hover styling.

The new-session screen is now a first-class standalone tile rather
than a separate route page. View registry distinguishes entity views
(chat, state-explorer) from standalone views (new-session); standalone
tiles carry entityUrl: null and render a tile chrome (header + split
menu + drop overlay) just like an entity tile. Both / and /entity/$
mount the same Workspace component and the URL <-> workspace sync
maps standalone tiles back to /.

Drag-and-drop covers all three sources (sidebar entity row, sidebar
new-session button, existing tile header) into the four edge
quadrants of any tile. openTile now focuses the freshly-created
tile in all paths so drop-to-side gives immediate visual feedback
and the URL follows. Multiple new-session tiles can coexist via
drag (the click flow keeps focus-existing-or-replace semantics).

The SplitMenu's view rows render inline with [->][down] icon
buttons for split-this-view-to-the-side; entity-only items (Inspect,
Pin, Fork, Kill, Copy URL) and "Close tile" (when sole tile) are
hidden contextually. The State Explorer's draggable divider switched
to the shared Splitter component for visual consistency.

Persistence layer: SCHEMA_VERSION bumped to v2; pruneNode keeps
standalone tiles intact. Layout codec encodes standalone tiles as
".viewId" (empty entity-path segment) so layout links can carry
mixed standalone+entity workspaces.

Co-authored-by: Cursor <[email protected]>