feat: improve code sidebar workflow

[Herbertlyw]

What changed

• add a fullscreen editor mode and drag-to-hide behavior at the right edge
• add file-type-aware icons and keep the new-tab action pinned in the tab bar
• preserve file-tree expansion and scroll position while switching files or tabs, without remount flicker
• add real file/folder creation, folder-scoped import, recursive deletion, and immediate tab/tree cleanup after deletion
• complete the new workflow copy across the existing locale set

Demo

Recorded from commit 2b10015015dc5907c545c4fa10861f150fceb2e0 using the packed dsh-better-sidebar@0.16.1 tarball in a fresh scratch DSH web profile. The recording is keyless, so it does not include a model round; the editor and filesystem operations shown are backed by the real local DSH services.

Verification

• pnpm exec vitest run — 1,039 passed, 9 skipped
• pnpm typecheck
• pnpm build
• pnpm check:consumer-types
• pnpm test:mount — 16 passed