Pure restructure — no behavior change. Extracts FileTree, FileEditor,
FilesToolbar, useFilesApi hook, and tree utilities into sibling files
under canvas/src/components/tabs/FilesTab/. Top-level FilesTab.tsx is
now 240 lines (glue + confirmations); re-exports buildTree/TreeNode so
the existing import path and tests remain stable.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
