Show missing dependencies in dependency graph instead of failing

When dependencies are not cached on the server (common since we removed
proactive caching), the dependency graph now:
- Continues resolving what it can find
- Shows missing dependencies in a separate section with amber styling
- Displays the constraint and which package required them
- Updates the header stats to show "X cached • Y not cached"

This provides a better user experience than showing an error when
some dependencies haven't been downloaded yet.

frontend/src/components/DependencyGraph.tsx

@@ -244,7 +244,11 @@ function DependencyGraph({ projectName, packageName, tagName, onClose }: Depende
             <span>{projectName}/{packageName} @ {tagName}</span>
             {resolution && (
               <span className="graph-stats">
-                {resolution.artifact_count} packages • {formatBytes(resolution.total_size)} total
+                {resolution.artifact_count} cached
+                {resolution.missing && resolution.missing.length > 0 && (
+                  <span className="missing-count"> • {resolution.missing.length} not cached</span>
+                )}
+                 • {formatBytes(resolution.total_size)} total
               </span>
             )}
           </div>
@@ -307,6 +311,22 @@ function DependencyGraph({ projectName, packageName, tagName, onClose }: Depende
           )}
         </div>
 
+        {resolution && resolution.missing && resolution.missing.length > 0 && (
+          <div className="missing-dependencies">
+            <h3>Not Cached ({resolution.missing.length})</h3>
+            <p className="missing-hint">These dependencies are referenced but not yet cached on the server.</p>
+            <ul className="missing-list">
+              {resolution.missing.map((dep, i) => (
+                <li key={i} className="missing-item">
+                  <span className="missing-name">{dep.project}/{dep.package}</span>
+                  {dep.constraint && <span className="missing-constraint">@{dep.constraint}</span>}
+                  {dep.required_by && <span className="missing-required-by">← {dep.required_by}</span>}
+                </li>
+              ))}
+            </ul>
+          </div>
+        )}
+
         {hoveredNode && (
           <div className="graph-tooltip">
             <strong>{hoveredNode.project}/{hoveredNode.package}</strong>