ui: offer Terminal and Desktop on every Keiki chat; the platform wakes the sandbox on open

[devin-ai-integration]

Summary

Terminal and Desktop were missing from the surface picker on Keiki conversations whose agent had never run a sandbox tool (e.g. api:5e9ee634-…:5d26257a… showed only "Conversations"): Comet probed GET …/terminal and hid both rows unless the platform had a sandbox on record. With Mail-0/sms-kit#984 the platform creates or wakes the sandbox when a terminal/desktop is opened, so the probe is gone and the rows are always offered — a click is what wakes the machine.

• shell.rs: keiki_terminal_available, check_keiki_terminal, terminal_offered removed; Terminal row is unconditional, desktop_offered() is just is_keiki_chat(active_chat). Browser keeps its own live-handoff probe (check_keiki_browser) — a different runtime, untouched.
• keiki-api: terminal_available removed; open_desktop returns Result<DesktopViewer, Error> (no None-on-404 branch).
• desktop.rs: DesktopEvent::Gone and the EventEmitter impl removed; an open failure just closes the tab with the platform's message, as other errors already did.

Net −94 lines.

Link to Devin session: https://app.devin.ai/sessions/4bd4a708feed454e9aa48d0b31cc6b4a
Open in Devin Desktop: https://app.devin.ai/desktop/session/4bd4a708feed454e9aa48d0b31cc6b4a?variant=devin
Requested by: @MrgSub

[devin-ai-integration]

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