ifcviewer-web: frame the first model only on load, matching desktop

applyCachedModel already does a one-time viewAll gated by initial_view_applied_
(desktop + web) — it frames the FIRST model loaded and never re-frames as more
arrive. Two web-only divergences from the desktop viewer, fixed here:

- loadSidecarMetadataWeb called viewAll() AGAIN, unconditionally, per model, so
  every federated model that streamed in yanked the camera back to fit the whole
  scene. Drop the redundant call; the shared gate handles first-model framing.

- Nothing ever cleared initial_view_applied_. On web the embedded sample sets it
  at startup, so after clear_scene_c (a ?model= federation load) the gate was
  already tripped and the loaded models never got framed — the camera stayed on
  the prior view. (The redundant per-model viewAll above masked this until it was
  removed.) Clear the flag in resetScene: a fresh scene should auto-frame its
  first model — correct for desktop fresh-opens too.

113/113 desktop + 6/6 web smoke pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/ViewportCore.cpp

@@ -111,6 +111,11 @@ void ViewportCore::removeModel(uint32_t model_id) {
 void ViewportCore::resetScene() {
     for (auto& [mid, m] : models_gpu_) releaseWgpuModelGpuData(m, pool_);
     models_gpu_.clear();
+    // A fresh scene should auto-frame its first model. Without this the flag
+    // stays set from the previous scene (on web, the embedded sample sets it at
+    // startup), so the next load — e.g. a ?model= federation after clear_scene_c
+    // — would never get framed.
+    initial_view_applied_ = false;
     host_->requestFrame();
 }
 
@@ -3587,7 +3592,11 @@ void ViewportCore::loadSidecarMetadataWeb(int source_id, std::string source_labe
                                             mit->second.deferred_raw_size    = dr;
                                         }
                                     }
-                                    viewAll();
+                                    // NOTE: no viewAll() here — applyCachedModel
+                                    // already frames the FIRST model (gated by
+                                    // initial_view_applied_), matching desktop.
+                                    // Reframing per model would jump the camera
+                                    // as each federated model streams in.
                                     host_->requestFrame();
                                     Log::info() << "ifcviewer-web: loaded sidecar (" << source_label
                                                 << ", id " << mid << ", " << n_meshes << " meshes, "