docs(FR-2571): address PR #6631 review feedback (excl. model store)

Addresses review comments on PR #6631 (Allowed client IPs / login screenshots) across all 4 languages. Model-store items are intentionally deferred to a separate change.

Text edits:
- header: rewrite V2 API wording (both Allowed client IPs and Original password notes) to explicit version gating — 'Backend.AI 26.4.0 and later'
- vfolder §10.1: add clarification that the top-bar project is auto-selected for project folders
- vfolder: rewrite write_content description as a per-folder (vfolder) RBAC permission, not a storage-host setting (verified via FolderExplorerModal.tsx — reads vfolder_node.permissions)
- vfolder §10.3: describe folder rename as available only inside the detail drawer
- vfolder §10.7: remove empty 'Folder Categories' heading
- overview §3.3: add Project page row to the accessible-menu table

Screenshots recaptured (EN + copied to ja/ko/th):
- menu_collapse.png (Fig 7.2 sidebar — no customer-specific plugins)
- data_page.png (Fig 10.1 / folder list with Controls merged into Name)
- vfolder_create_modal.png (current folder-creation modal)
- rename_vfolder.png (rename inside folder detail drawer)
- move_to_trash.png (new Ant Design Modal, replacing Popconfirm)
- vfolder_delete_dialog.png (current Delete Forever modal)
- sessions_page.png (Fig 11.1 current Sessions page)
- launch_session_resource_2.png (Cluster Mode step)
- launch_session_data.png (Data & Storage step)

Remaining TODO: header.png Login Session Timer (dev env has enableExtendLoginSession=false; timer not visible without backend config change).

Applied identically across en/ko/ja/th with structural parity.

Author: nowgnuesLee