A machine's shell page says a detached session's stop time and picks it up where it was #10546

Split from #10457 (leaving is not losing).

Closing the tab already leaves the tmux session running. The page does not say so.

Done when:

• [x] When the last peer goes, the session is detached rather than ended, and the machine page (src/web/pages/machines.ts) shows that machine's line as "detached, stops at HH:MM unless you come back" (the forge's time format, computed from the idle setting and last activity).
• [x] Opening the shell again joins the same session with its scrollback; the line goes back to "attached".
• [x] Tests: a closed socket leaves the session alive and the page renders its stop time; reconnecting joins it rather than starting another.

bun scripts/design-lint.ts passes; before/after screenshot.

Reviewed pull #10654 at its head: ticked When the last peer goes, the session is detached rather than ended, and the machine page (src/web/pages/machines.ts) shows that machine's line as "detached, stops at HH:MM unless you come back" (the forge's time format, computed from the idle setting and last activity).; Opening the shell again joins the same session with its scrollback; the line goes back to "attached".; Tests: a closed socket leaves the session alive and the page renders its stop time; reconnecting joins it rather than starting another..