refactor(canvas): split 650-line FilesTab.tsx into focused components

[HongmingWang-Rabbit]

Summary

• Pure restructure of canvas/src/components/tabs/FilesTab.tsx (650 lines) into focused sibling files under canvas/src/components/tabs/FilesTab/.
• No behavior change. Top-level FilesTab.tsx stays at the same import path (240 lines, glue + confirmation dialogs) and re-exports buildTree / TreeNode so the existing buildTree.test.ts and SidePanel.tsx imports are unaffected.
• Preserves all dark-theme classes, aria/keyboard handlers, fetch URLs/methods/bodies, error handling, and the in-panel ConfirmDialog-style banners (no confirm()/alert()/prompt() introduced).

Files

• FilesTab.tsx — 240 lines (was 650): state orchestration, layout, inline confirm banners, re-exports.
• FilesTab/FilesToolbar.tsx — 77 lines: root selector, New/Upload/Export/Clear/Refresh buttons.
• FilesTab/FileTree.tsx — 115 lines: recursive FileTree + TreeItem.
• FilesTab/FileEditor.tsx — 112 lines: header, save/download buttons, textarea with Ctrl/Cmd+S and Tab handling.
• FilesTab/useFilesApi.ts — 172 lines: GET/PUT/DELETE API wrapper, directory expansion, bulk upload/download/delete.
• FilesTab/tree.ts — 86 lines: FileEntry, TreeNode, getIcon, buildTree utility.

Test plan

• cd canvas && npm run build — clean
• cd canvas && npm test -- --run — 357/357 pass (incl. existing buildTree tests)
• wc -l — top file 240 (≤250), largest sibling 172 (≤300)
• grep -n "confirm(\|alert(\|prompt(" FilesTab* — 0 matches
• Manual smoke test: open a workspace's Files tab, browse tree, edit/save/delete a file, upload folder, download single + export all

[HongmingWang-Rabbit]

7-Gate Verification: PASS

1. CI — all 6 jobs green (Canvas, Platform, MCP, Python Lint, E2E API Smoke, Shellcheck)
2. Checkout + build — npm run build succeeds (283kB first-load JS, 5 static pages)
3. Tests — vitest run 357/357 pass across 19 test files
4. Security audit — clean:
   • No dangerouslySetInnerHTML / unescaped content in new JSX
   • All fetch calls in useFilesApi.ts go through the api.* helper (auth layer preserved)
   • No leaked secrets
   • Path handling identical to pre-refactor (URL interpolation on ${workspaceId}/files/${path}) — no new surface
5. Design fit — zero native dialogs (grep on confirm(|alert(|prompt( in FilesTab* returns nothing), no light-theme color classes introduced in FilesTab/, 'use client' present on the 3 hook-using .tsx files (FileEditor, FileTree, FilesToolbar) and on useFilesApi.ts
6. Line-level review — pure extraction: fetch URLs/methods preserved (GET /workspaces/:id/files?root=&depth=, PUT, DELETE), needsRestart marker preserved on write/delete/upload paths, keyboard/aria attributes preserved, buildTree + TreeNode re-exported from root FilesTab.tsx to keep external imports working, no dead exports
7. Manual UI test — e2e/filestab-smoke.spec.ts passes: workspace created, detail panel opens, Files tab renders FilesToolbar (+ New / Upload), FileTree (config.yaml), and FileEditor (empty-state) — all three split components visible

Change is NOT noteworthy (pure file-split refactor, no auth/billing/data-deletion touched). Merging as merge-commit.