feat(M8): workspace isolation + SSH bottom file panel + context menu + monitor polish
- dual dockview workspaces (terminal/ssh), rail mode entries, sidebar filtered by mode
- SshBottomPanel: file browser below terminal, draggable divider; sftp.css finally imported (was orphaned since M4)
- file context menu: refresh/open/download/upload/rename/mkdir/copy-path/permissions/delete
- monitor restyle: memory amber bar, disk rows, net rx/tx inline (toFixed 4)
- completion fixes: Enter accepts; char-loss fixed by full-line rewrite (Ctrl+U) + echo-suppress ref
- chmod no-op fix: bitsToOctal concatenated decimal (329) instead of octal (511); chown only on change
- popups (layout menu/broadcast popover/file context menu) mount to body - dockview render overlay isolation clipped them under the sidebar
- rail slimmed: split + template actions folded into layout menu; log toolbar hover-only
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