fix(eeschema): wasm dialog positioning (web .window CSS) + collab SCH_SHAPE add via coroutine fiber stack

Thrust A — dialogs render top-left with OK clipped in the WEB app (not the
test harness): root cause was the React shell missing the .window /
.window-canvas CSS that wx.js relies on (it positions each dialog div via
inline left/top, which need position:absolute). Added the rules to
web/apps/frontend/src/index.css. Native draw-text now works end-to-end;
symbol/power choosers render (placing still blocked by absent libraries).

Thrust B — collab apply of a newly-added SCH_SHAPE trapped in KiCad core
(SCH_COMMIT::Push CHT_ADD -> GAL view->Add, an asyncify invoke_* mis-dispatch)
because doApply ran off a fiber stack. doApply now runs inside a COROUTINE so
it executes on a libcontext fiber, the same context native draws use; the add
dispatches correctly. Re-enabled the SCH_SHAPE converter (rect/circle). Added
thirdparty/libcontext to the embind include path (tool/coroutine.h needs it).
Verified two-tab: rectangle + circle drawn in tab A sync + render in tab B.
Extended eeschema-collab.spec.ts apply test with a SCH_SHAPE add.

All changes root-repo only; kicad and wxwidgets forks untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/kicad/build-kicad-target.sh

@@ -405,6 +405,8 @@ if [ -f "${EMBIND_SRC}" ]; then
     KICAD_INCLUDES+=" -I${KICAD_DIR}/thirdparty/nanodbc"
     KICAD_INCLUDES+=" -I${KICAD_DIR}/thirdparty/picosha2"
     KICAD_INCLUDES+=" -I${KICAD_DIR}/thirdparty"
+    # libcontext.h lives one level deeper; tool/coroutine.h does #include <libcontext.h>
+    KICAD_INCLUDES+=" -I${KICAD_DIR}/thirdparty/libcontext"
     KICAD_INCLUDES+=" -I${SYSROOT}/include"
     # KiCad requires C++20 for concepts
     em++ -std=c++20 -c ${EXTRA_FLAGS} ${WX_CXXFLAGS} ${KICAD_INCLUDES} "${EMBIND_SRC}" -o "${EMBIND_OBJ}"