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>
       )}

ui/src/pages/Projects.tsx

@@ -1,4 +1,4 @@
-import { useEffect } from "react";
+import { useEffect, useMemo } from "react";
 import { useQuery } from "@tanstack/react-query";
 import { projectsApi } from "../api/projects";
 import { useCompany } from "../context/CompanyContext";
@@ -22,11 +22,15 @@ export function Projects() {
     setBreadcrumbs([{ label: "Projects" }]);
   }, [setBreadcrumbs]);
 
-  const { data: projects, isLoading, error } = useQuery({
+  const { data: allProjects, isLoading, error } = useQuery({
     queryKey: queryKeys.projects.list(selectedCompanyId!),
     queryFn: () => projectsApi.list(selectedCompanyId!),
     enabled: !!selectedCompanyId,
   });
+  const projects = useMemo(
+    () => (allProjects ?? []).filter((p) => !p.archivedAt),
+    [allProjects],
+  );
 
   if (!selectedCompanyId) {
     return <EmptyState icon={Hexagon} message="Select a company to view projects." />;
@@ -47,7 +51,7 @@ export function Projects() {
 
       {error && <p className="text-sm text-destructive">{error.message}</p>}
 
-      {projects && projects.length === 0 && (
+      {!isLoading && projects.length === 0 && (
         <EmptyState
           icon={Hexagon}
           message="No projects yet."
@@ -56,7 +60,7 @@ export function Projects() {
         />
       )}
 
-      {projects && projects.length > 0 && (
+      {projects.length > 0 && (
         <div className="border border-border">
           {projects.map((project) => (
             <EntityRow