Live mode: an AI assistant acts on the open project while the user watches

A new setting, Configurer QElectroTech > Général > "Autoriser un assistant
IA à agir sur le projet ouvert", off by default. Off, nothing changes:
no channel is opened. On, every start shows a warning first -- Continuer,
Pas pour cette session, or Désactiver -- and nothing can connect until
the user answers Continuer. It waits for any other start-up question to
be answered, so the two never stack.

Accepted, LiveServer opens a local socket only the user's account can
use, with a random name and token written to live-session.json in the
data folder for the qet MCP server, and removed when the channel closes.
One JSON request per line: status (project, folio on screen, selection,
last undo step, stored scripts), run_script and run_stored. Requests are
queued out of the socket handler before they run (the lesson of PR #861).

Each run is one undo step named "Assistant : <name>". The status bar
shows the mode and the assistant's last action with a ✓ or ✗ and the
time, and an Arrêter button that closes the channel for the session.
Unticking the setting closes it at once.

QetScripting::runSource() runs script text and, for a live run, returns
what qet.log() wrote, the error with its line and the undo step instead
of showing boxes; qet.showMessage() is logged rather than opening a box
nobody asked for.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmake/qet_compilation_vars.cmake

@@ -928,6 +928,8 @@ list(APPEND QET_SRC_FILES
   ${QET_DIR}/sources/scripting/scriptlibrary.h
   ${QET_DIR}/sources/scripting/scriptmanagerdialog.cpp
   ${QET_DIR}/sources/scripting/scriptmanagerdialog.h
+  ${QET_DIR}/sources/scripting/liveserver.cpp
+  ${QET_DIR}/sources/scripting/liveserver.h
 )
 
 if(QET_SPACEMOUSE_ENABLED)