Add archive project button and filter archived projects from selectors

- Add "Archive project" / "Unarchive project" button in the project
  configuration danger zone (ProjectProperties)
- Filter archived projects from the Projects listing page
- Filter archived projects from NewIssueDialog project selector
- Filter archived projects from IssueProperties project picker
  (keeps current project visible even if archived)
- Filter archived projects from CommandPalette
- SidebarProjects already filters archived projects

Co-Authored-By: Paperclip <noreply@paperclip.ing>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/pages/ProjectDetail.tsx

@@ -274,6 +274,21 @@ export function ProjectDetail() {
     onSuccess: invalidateProject,
   });
 
+  const archiveProject = useMutation({
+    mutationFn: (archived: boolean) =>
+      projectsApi.update(
+        projectLookupRef,
+        { archivedAt: archived ? new Date().toISOString() : null },
+        resolvedCompanyId ?? lookupCompanyId,
+      ),
+    onSuccess: (_, archived) => {
+      invalidateProject();
+      if (archived) {
+        navigate("/projects");
+      }
+    },
+  });
+
   const uploadImage = useMutation({
     mutationFn: async (file: File) => {
       if (!resolvedCompanyId) throw new Error("No company selected");
@@ -476,6 +491,8 @@ export function ProjectDetail() {
             onUpdate={(data) => updateProject.mutate(data)}
             onFieldUpdate={updateProjectField}
             getFieldSaveState={(field) => fieldSaveStates[field] ?? "idle"}
+            onArchive={(archived) => archiveProject.mutate(archived)}
+            archivePending={archiveProject.isPending}
           />
         </div>
       )}