feat(desktop/terminal): connect-phase UX and SSH split-duplicate (#319)

[agentfleets]

Closes #319

The remaining tail of the terminal-lifecycle issue: connecting-phase indication with cancel/timeout, and split-duplicate for SSH sessions.

What

• Connecting phases + cancel + timeout (desktop/src/terminal/ConnectForm.tsx, desktop/src-tauri/src/ssh.rs): ssh_connect now emits ssh-connect-progress ticks at each handshake stage (tcp → auth → channel), tagged with a frontend-minted connect_id (same pattern as the SFTP transfer_id). The connect button label follows the live phase; a Cancel button abandons the attempt; a 20 s ceiling (CONNECT_TIMEOUT_MS) abandons stuck handshakes with a timeout error. An abandoned attempt that connects late has its orphaned session closed (sshClose) instead of leaking a shell nobody renders. The form's close-Cancel is replaced by the attempt-Cancel while busy, so there is always exactly one Cancel button.
• Split-duplicate an SSH session (desktop/src/terminal/TerminalPanel.tsx, ssh.rs): new ssh_duplicate command opens a second interactive shell channel on the existing connection's shared russh Handle — no re-authentication, works for ad-hoc and key-auth tabs alike. Splitting with an SSH tab active duplicates it into the new tile; local tabs keep spawning a fresh local shell. The actor-spawn tail of ssh_connect was extracted into spawn_shell_session, shared verbatim by both commands. The shared-orientation split model is kept — no nested split tree.

Why

• A hanging handshake looked frozen: no phase, no cancel, no timeout (issue item 2).
• Split always spawned a new local shell even from SSH tabs (issue item 8).

How verified

• cd desktop && npm run build (sync:tokens + tsc --noEmit + vite build) — green.
• bash scripts/lint-desktop-tokens.sh — clean (86 off-token, all ≤ baseline, no phantom tokens; no CSS touched).
• git diff origin/main --stat — only the 6 intended files.
• Rust changes are CI-verified only (no local toolchain). Kept minimal: the extracted actor loop is byte-identical to the previously-compiling ssh_connect tail, and the one newly-named type (russh::Channel<client::Msg>) was checked against the russh 0.62.0 crate sources (Handle::channel_open_session returns exactly that).

i18n: term.phaseTcp / term.phaseAuth / term.phaseChannel / term.connectTimeout added in both en and zh; no other user-visible strings.

[physercoe]

APPROVE. Connect-phase UX + SSH split-duplicate implemented cleanly. ssh_duplicate reuses the shared connection Arc<Handle> (no re-auth), per-attempt filtering guards stale phase ticks, all new strings have en+zh. Verified no conflict with the ConPTY windowsPty fix on main. Nit (non-blocking): the phase listener is not unlisten()ed on the cancel/timeout paths, so a truly-hung sshConnect invoke would leak a listener — harmless given per-attempt filtering.