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.css

@@ -55,6 +55,10 @@
   font-size: 0.8125rem;
 }
 
+.missing-count {
+  color: #f59e0b;
+}
+
 .close-btn {
   background: transparent;
   border: none;
@@ -314,6 +318,63 @@
   font-size: 0.75rem;
 }
 
+/* Missing Dependencies */
+.missing-dependencies {
+  border-top: 1px solid var(--border-primary);
+  padding: 16px 20px;
+  background: rgba(245, 158, 11, 0.05);
+  max-height: 200px;
+  overflow-y: auto;
+}
+
+.missing-dependencies h3 {
+  margin: 0 0 8px 0;
+  font-size: 0.875rem;
+  font-weight: 600;
+  color: #f59e0b;
+}
+
+.missing-hint {
+  margin: 0 0 12px 0;
+  font-size: 0.75rem;
+  color: var(--text-muted);
+}
+
+.missing-list {
+  list-style: none;
+  padding: 0;
+  margin: 0;
+  display: flex;
+  flex-wrap: wrap;
+  gap: 8px;
+}
+
+.missing-item {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  background: var(--bg-tertiary);
+  border: 1px solid rgba(245, 158, 11, 0.3);
+  border-radius: var(--radius-sm);
+  padding: 4px 8px;
+  font-size: 0.75rem;
+}
+
+.missing-name {
+  font-family: 'JetBrains Mono', monospace;
+  color: var(--text-secondary);
+}
+
+.missing-constraint {
+  color: var(--text-muted);
+  font-family: 'JetBrains Mono', monospace;
+}
+
+.missing-required-by {
+  color: var(--text-muted);
+  font-size: 0.6875rem;
+}
+
 /* Responsive */
 @media (max-width: 768px) {
   .dependency-graph-modal {

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>

frontend/src/types.ts

@@ -433,6 +433,13 @@ export interface ResolvedArtifact {
   download_url: string;
 }
 
+export interface MissingDependency {
+  project: string;
+  package: string;
+  constraint: string | null;
+  required_by: string | null;
+}
+
 export interface DependencyResolutionResponse {
   requested: {
     project: string;
@@ -440,6 +447,7 @@ export interface DependencyResolutionResponse {
     ref: string;
   };
   resolved: ResolvedArtifact[];
+  missing: MissingDependency[];
   total_size: number;
   artifact_count: number;
 }