ifcviewer: de-Qt ViewportWindow public API (QString → std::string)

Take QString out of ViewportWindow's outward-facing surface so it can
eventually move into a Qt-free ViewportCore:

  void     queueLoadSidecar(const QString&)    →  (const std::string&)
  uint32_t loadSidecar(const QString&)         →  (const std::string&)
  QString  cameraString() const                →  std::string …
  void     captureNextFrameToPng(const QString&, bool)
                                               →  (const std::string&, bool)
  void     setHudText(const QString&)          →  (const std::string&)

Internal members also moved off QString:
  std::deque<QString> pending_sidecars_     →  std::deque<std::string>
  QString             pending_screenshot_path_ →  std::string

Implementation strategy: convert at the boundary where ViewportWindow
still leans on Qt internals — `loadSidecar` bridges to QString once
for QFile/QDir/QFileInfo path handling; the screenshot save path
constructs a QString locally for QImage::save; the OverlayRenderer's
HUD setter still takes QString so setHudText converts before calling
through. Each of those bridges goes away when ViewportCore lands and
OverlayRenderer / SceneLoader / SidecarBuilder get their own de-Qt
sweeps. cameraString now produces its CSV via snprintf — no QString
ever instantiated.

Bonsai-side updates (compile-only):
  ifcviewer-minimal/main.cpp — queueLoadSidecar / captureNextFrameToPng
                              callers add .toStdString() on the QString
                              parser result
  modules/viewport/View.cpp  — setHudText callers add .toStdString() to
                              their `QString::arg(...)` formatter chains;
                              two `QString()` empty sentinels become
                              `std::string()`
  Measurement.cpp            — same pattern, two setHudText sites
  ifcviewer/LengthMeasurement.cpp — same, three sites

The cameraString string-streaming fix-up in ViewportWindow.cpp drops
the temporary .toUtf8().constData() bridge from #82 — Log::Stream's
std::string overload now handles it directly.

Builds: desktop / bonsai / web all green. Tests 100/100. Closes #80.

src/bonsaiviewer/Measurement.cpp

@@ -662,7 +662,7 @@ void LengthMeasurement::clear(ViewportWindow& vp) {
     vp.setOverlayPoints({}, 0,0,0,0, 0, 0,0,0,0, 0);
     vp.setOverlayLines({});
     vp.setOverlayLabels({});
-    vp.setHudText(QString());
+    vp.setHudText(std::string());
 }
 
 void LengthMeasurement::onPick(ViewportWindow& vp, int x, int y, bool /*alt*/) {
@@ -805,7 +805,7 @@ void LengthMeasurement::rebuildOverlay(ViewportWindow& vp) {
 
     vp.setOverlayLines(groups);
     vp.setOverlayLabels(labels);
-    vp.setHudText(formatReadout());
+    vp.setHudText(formatReadout().toStdString());
 }
 
 namespace {
@@ -1029,7 +1029,7 @@ void LengthMeasurement::rebuildLaserOverlay(ViewportWindow& vp) {
     pushDots(vp, std::vector<float>(wp, wp + 3));
     vp.setOverlayLines(groups);
     vp.setOverlayLabels(labels);
-    vp.setHudText(hud_lines.join('\n'));
+    vp.setHudText(hud_lines.join('\n').toStdString());
 }
 
 QString LengthMeasurement::formatReadout() const {