fix: reorder worktree groups by dragging their header

[omartelo]

What

Reordering sessions stopped at the group boundary. A card moved freely inside its own group, but the groups themselves sat in whatever order they happened to open in — so with several worktrees open, the one you were done with kept its place at the top and there was no way to move it. Drag a group by its header (the divider carrying the worktree's name) and the whole block moves with it, cards and pull request card included.

A sidebar with a single group draws no header and is unchanged.

How

A group has no row anywhere: groupByWorktree derives it from the flat session list, bucketing by session.path and ordering by first appearance. Moving a group therefore means moving its whole block of ids inside that one list.

• orderGroups(groups, keys) builds that flat id order, each group keeping its internal order.
• SessionSidebar wraps the groups in an outer DndContext + SortableContext over the group keys; the drop commits reorderSessions(projectId, orderGroups(groups, keys)).
• SessionGroup becomes a sortable of that outer context but takes the drag from its header alone, so the per-group card context and the group context never contend for the same pointer. It is disabled when there is no header — a lone group has nothing to reorder.
• ROOT_GROUP_KEY stands in for the root group's empty path, which a dnd-kit sortable id cannot be. It replaces the same "__root__" literal the React key already spelled out.

A key naming no group contributes no ids, so a group closed mid-drag makes the order short and reorderSessions' id-set check drops it whole rather than taking that group's sessions with it.

Known ceiling

A pinned session still carries its group to the head of the list (#180). Dropping another group above a pinned group snaps back — the pin stays the one order a drag cannot beat. Recorded in the CHANGELOG entry rather than worked around.

Test plan

• pnpm test — 652 pass, 3 new covering orderGroups (block move, root key, unknown key)
• tsc --noEmit clean, biome check clean (no new warnings)
• vite build succeeds
• CHANGELOG.md [Unreleased] entry lands here
• By hand in task dev — the frontend gate is node-only, so a render path is not covered: open two worktrees, drag a group header, confirm the block moves and that clicking a card still selects it

Follow-up

Filed as #188: the header runs baseName over the checkout path, so a worktree named feat/x renders as x and collides with fix/x.