v0.9.7: a turn that cannot hang, a session list that reads nothing twice, and PDFs

- consent: on_permission / on_ask_user wait at most two / five minutes, and in
  the background (a phone off screen, a window that is gone) the answer is
  refused at once with a reason -- a dialog nobody can see blocked the worker
  for good, which is what "the app hangs while the agent works" looked like.
- finishTurn() no longer joins the thread it is called from; the event loop
  reaps the Turn instead.
- AndroidTools: the hop to the GUI thread pings first and gives up after 3 s
  instead of waiting forever on a busy GUI thread.
- the session list stopped reading every transcript: cst_agent_store_info()
  counts newlines in 64 KiB blocks (first_user only within the first 256 KiB),
  and AgentCore::sessions() reuses a row while (mtime, size) is unchanged.
- session_search gets a budget (max_bytes 32 MB, max_seconds 5 s), skips lines
  over 64 KiB, and says when the budget stopped the walk.
- the header shows the phase ("shell: make -j8"), so a long turn no longer
  looks like a stuck one.
- md_to_pdf: the front end's own Qt tool (owner app:qt, every platform):
  markdown or a workspace .md/.html becomes an A4 PDF with 15 mm margins,
  paginated by hand, written into the workspace and delivered as a file card.
  No PrintSupport, so the phone gets it too.
- --uitest runs md_to_pdf through the registry with Chinese text and checks the
  bytes are a PDF; app/CMakeLists.txt no longer drops src/web_md.c from the
  standalone engine build, which the window needs to render markdown.

Version 0.9.7.

app/CMakeLists.txt

@@ -43,8 +43,10 @@ if(NOT TARGET cstcore)
   list(REMOVE_ITEM CST_APP_CORE_SRC
     "${CST_APP_ROOT}/src/net.c"
     "${CST_APP_ROOT}/src/http_srv.c"
-    "${CST_APP_ROOT}/src/webui.c"
-    "${CST_APP_ROOT}/src/web_md.c")
+    "${CST_APP_ROOT}/src/webui.c")
+  # web_md.c stays in: the window renders markdown with it (AgentCore::
+  # renderMarkdown) and the md_to_html tool needs it. Only the HTTP server
+  # is left out.
   if(EXISTS "${CST_APP_ROOT}/.gen/models_builtin.inc")
     include_directories("${CST_APP_ROOT}/.gen")
   endif()
@@ -100,6 +102,7 @@ if(CST_APP_GUI)
       src/main.cpp
       src/core/AgentCore.cpp
       src/core/ToolProvider.cpp
+      src/core/PdfTools.cpp
       src/ui/UiTest.cpp
       src/ui/ThemeIcons.cpp
       src/ui/SessionImages.cpp