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.

backend/app/dependencies.py

@@ -42,6 +42,7 @@ from .schemas import (
     ResolvedArtifact,
     DependencyResolutionResponse,
     DependencyConflict,
+    MissingDependency,
     PaginationMeta,
 )
 
@@ -690,6 +691,8 @@ def resolve_dependencies(
 
     # Track resolved artifacts and their versions
     resolved_artifacts: Dict[str, ResolvedArtifact] = {}
+    # Track missing dependencies (not cached on server)
+    missing_dependencies: List[MissingDependency] = []
     # Track version requirements for conflict detection
     version_requirements: Dict[str, List[Dict[str, Any]]] = {}  # pkg_key -> [(version, required_by)]
     # Track visiting/visited for cycle detection
@@ -773,12 +776,15 @@ def resolve_dependencies(
             )
 
             if not resolved_dep:
+                # Dependency not cached on server - track as missing but continue
                 constraint = dep.version_constraint or dep.tag_constraint
-                raise DependencyNotFoundError(
-                    dep.dependency_project,
-                    dep.dependency_package,
-                    constraint,
-                )
+                missing_dependencies.append(MissingDependency(
+                    project=dep.dependency_project,
+                    package=dep.dependency_package,
+                    constraint=constraint,
+                    required_by=pkg_key,
+                ))
+                continue
 
             dep_artifact_id, dep_version, dep_size = resolved_dep
             _resolve_recursive(
@@ -828,6 +834,7 @@ def resolve_dependencies(
             "ref": ref,
         },
         resolved=resolved_list,
+        missing=missing_dependencies,
         total_size=total_size,
         artifact_count=len(resolved_list),
     )