fix(win): graceful shutdown, one startup-mode control, and a stray-\r bug

Windows-only changes, all found by actually running the previous session's
work rather than by review alone:

- CTRL_CLOSE_EVENT/LOGOFF/SHUTDOWN handler (platform.py, ctypes
  SetConsoleCtrlHandler) so closing a console window, signing off, or a
  system shutdown runs the daemon's real _shutdown() instead of Windows
  just ending the process — closing WebRTC sessions and any in-flight
  ffmpeg transcode instead of orphaning it. `taskkill /F` itself stays
  uncatchable (like SIGKILL), so autostart_run() now spawns with
  CREATE_NEW_PROCESS_GROUP instead of DETACHED_PROCESS and autostart_end()
  tries CTRL_BREAK_EVENT against the recorded pid first, falling back to
  the hard kill only if that doesn't stop it in time.

- Replaced the Node page's two independent autostart/service-mode toggles
  with one "start automatically" select (off / at sign-in / as a
  background service). The old pair let both be active at once — starting
  the daemon twice, at boot and at sign-in — and their layout broke
  wrapping inside .node-service's flex row. The new control always removes
  whichever mechanism is active before installing the target; platform.py's
  service_install() does the same on the CLI side. The "background
  service" option disables itself (with a hint pointing at the CLI) when
  running unpackaged, since service-mode.ps1/service.ps1/firewall.ps1 all
  assume an installed build's layout — verified live rather than assumed
  by actually running those scripts unelevated.

- findNodeBinary() no longer bakes a stray \r into resolved paths. Found by
  rebooting after enabling per-user autostart: where.exe listed two
  matches, and stdout.trim().split('\n')[0] only strips the whole string's
  ends, leaving line one's own trailing \r attached — which landed inside
  the Startup .vbs's quoted path and broke it with "Unterminated string
  constant" at boot. Fixed by splitting on \r?\n and trimming every line.

- Dependency audit for the Windows installer (docs/WINDOWS-PORT.md): no
  VC++ Redistributable needed, confirmed by inspecting the built
  node-runtime's actual import table rather than assuming. New
  docs/windows-build.md: a concise clone-to-installer build guide.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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