Centre the viewer status text so it clears the pane labels

#status sat at top:8px/left:12px, directly under the "Coronal" pane label at
top:10px/left:10px, so "Downloading volume..." overlapped it. The four pane
labels own the corners, so the status moves to the centre of the canvas as a
pill with a backdrop, which is also where a loading indicator reads best.

Two follow-ons: #status:empty is transparent so no empty pill floats over the
image when idle, and the download/decode timing shows for 3.5s then fades
rather than sitting permanently on top of the volume. setStatus() clears the
fade class so a re-render is visible again.

Also moved the viewer_copy import inside the DOWNCAST_VIEWER branch. It ran
unconditionally, so NV_GENERATE_DOWNCAST_VIEWER=0 still required nibabel to be
importable -- which defeats the point of an escape hatch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

viewer/niivue_embed.py

@@ -75,7 +75,22 @@ def _build_iframe_doc(cfg_json: str, container_id: str) -> str:
 <style>
   html, body {{ margin: 0; padding: 0; height: 100%; background: #0b1020; overflow: hidden; }}
   #{container_id} {{ display: block; width: 100%; height: 100%; background: #0b1020; }}
-  #status {{ position: absolute; top: 8px; left: 12px; color: #8c93ad; font: 12px -apple-system, BlinkMacSystemFont, sans-serif; pointer-events: none; }}
+  /* Centred rather than top-left: the four pane labels own the corners, and a
+     status pill at top-left collided with "Coronal". */
+  #status {{
+    position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%);
+    color: #cfd6ea; font: 12px/1.4 -apple-system, BlinkMacSystemFont, sans-serif;
+    text-align: center; white-space: nowrap;
+    padding: 8px 14px;
+    background: rgba(8, 16, 36, 0.78);
+    border: 1px solid rgba(170, 200, 255, 0.18);
+    backdrop-filter: blur(4px);
+    pointer-events: none;
+    opacity: 1; transition: opacity 400ms ease;
+  }}
+  /* No empty pill floating over the image once loading is done. */
+  #status:empty {{ opacity: 0; }}
+  #status.fade {{ opacity: 0; }}
   #err {{ position: absolute; bottom: 8px; left: 12px; right: 12px; color: #ff8d8d; font: 12px monospace; white-space: pre-wrap; pointer-events: none; }}
   /* Pane labels overlaid at the outer corner of each MPR quadrant */
   .pane-labels {{ position: absolute; inset: 0; pointer-events: none; }}
@@ -110,7 +125,11 @@ def _build_iframe_doc(cfg_json: str, container_id: str) -> str:
 const cfg = {cfg_json};
 const statusEl = document.getElementById('status');
 const errEl = document.getElementById('err');
-function setStatus(t) {{ if (statusEl) statusEl.textContent = t; }}
+function setStatus(t) {{
+  if (!statusEl) return;
+  statusEl.textContent = t;
+  statusEl.classList.remove('fade');
+}}
 function showErr(e) {{ if (errEl) errEl.textContent = String(e && e.stack || e); console.error(e); }}
 
 function b64ToBlobUrl(b64, mime) {{
@@ -235,7 +254,13 @@ function b64ToBlobUrl(b64, mime) {{
 
     nv.setSliceType(nv.sliceTypeMultiplanar);
     nv.updateGLVolume();
-    setStatus(statusEl && statusEl.dataset.timing ? statusEl.dataset.timing : '');
+    // Show the timing split briefly, then fade -- useful to see, not to keep.
+    if (statusEl && statusEl.dataset.timing) {{
+      setStatus(statusEl.dataset.timing);
+      setTimeout(() => {{ statusEl.classList.add('fade'); }}, 3500);
+    }} else {{
+      setStatus('');
+    }}
   }} catch (e) {{ showErr(e); }}
 }})();
 </script>
@@ -276,9 +301,13 @@ def render_viewer(
         # still crosses the wire compressed, but the browser inflates half as
         # many bytes and uploads half the texture, which is where the wait
         # actually was: download measured 1.5s, decode was the rest.
-        from utils.nifti_io import viewer_copy
+        vol_src = volume_path
+        if DOWNCAST_VIEWER:
+            # Imported here, not at module scope: with downcasting off the
+            # viewer must not depend on nibabel being importable at all.
+            from utils.nifti_io import viewer_copy
 
-        vol_src = viewer_copy(volume_path) if DOWNCAST_VIEWER else volume_path
+            vol_src = viewer_copy(volume_path)
         mask_src = mask_path  # masks are label maps, already integral
         vol_url = _file_url(vol_src)
         mask_url = _file_url(mask_src) if mask_src else None