fix(ui): unify sidebar controls and keyboard shortcuts

[antonreshetov]

Sidebar controls previously used space-specific labels, and the View menu could show the secondary sidebar as closed while AI chat was visible. Use shared visibility and toggle logic for toolbar controls and the system menu, replacing layout presets with Primary and Secondary Side Bar controls.

• Assign Cmd/Ctrl+B to the primary sidebar and Cmd+Option+B / Ctrl+Alt+B to the secondary sidebar.
• Make Cmd/Ctrl+L open or focus AI chat, and close the entire secondary sidebar when focus is already inside chat, restoring previous focus.
• Show existing shortcuts in tooltips, use a consistent chat icon, and update English/Russian labels and documentation.

Validation: 62 scoped tests passed; scoped ESLint, changed Vue component compilation, and locale synchronization passed.