branchtree: extract branch tree rendering from widgets (#1004)

Add a new branchtree package that specializes fliptree for rendering
branch trees with rich metadata (change IDs, worktrees, push status).

This extracts and generalizes the rendering logic previously embedded
in branch_select.go and log.go, providing a reusable Graph/Item model
with configurable styling.

Author: abhinav

internal/ui/branchtree/doc.go

@@ -0,0 +1,2 @@
+// Package branchtree specializes fliptree to branches.
+package branchtree