feat(ui): drag-to-reorder sidebar projects with persistent order

Add drag-and-drop reordering to sidebar project list using dnd-kit,
persisted per-user via localStorage. Use consistent project order in
issue properties, new issue dialog, and issue detail mention options.
Move projects section below Work section in sidebar.

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

ui/src/lib/project-order.ts

@@ -0,0 +1,70 @@
+import type { Project } from "@paperclip/shared";
+
+export const PROJECT_ORDER_UPDATED_EVENT = "paperclip:project-order-updated";
+const PROJECT_ORDER_STORAGE_PREFIX = "paperclip.projectOrder";
+const ANONYMOUS_USER_ID = "anonymous";
+
+type ProjectOrderUpdatedDetail = {
+  storageKey: string;
+  orderedIds: string[];
+};
+
+function normalizeIdList(value: unknown): string[] {
+  if (!Array.isArray(value)) return [];
+  return value.filter((item): item is string => typeof item === "string" && item.length > 0);
+}
+
+function resolveUserId(userId: string | null | undefined): string {
+  if (!userId) return ANONYMOUS_USER_ID;
+  const trimmed = userId.trim();
+  return trimmed.length > 0 ? trimmed : ANONYMOUS_USER_ID;
+}
+
+export function getProjectOrderStorageKey(companyId: string, userId: string | null | undefined): string {
+  return `${PROJECT_ORDER_STORAGE_PREFIX}:${companyId}:${resolveUserId(userId)}`;
+}
+
+export function readProjectOrder(storageKey: string): string[] {
+  try {
+    const raw = localStorage.getItem(storageKey);
+    if (!raw) return [];
+    return normalizeIdList(JSON.parse(raw));
+  } catch {
+    return [];
+  }
+}
+
+export function writeProjectOrder(storageKey: string, orderedIds: string[]) {
+  const normalized = normalizeIdList(orderedIds);
+  try {
+    localStorage.setItem(storageKey, JSON.stringify(normalized));
+  } catch {
+    // Ignore storage write failures in restricted browser contexts.
+  }
+  if (typeof window !== "undefined") {
+    window.dispatchEvent(
+      new CustomEvent<ProjectOrderUpdatedDetail>(PROJECT_ORDER_UPDATED_EVENT, {
+        detail: { storageKey, orderedIds: normalized },
+      }),
+    );
+  }
+}
+
+export function sortProjectsByStoredOrder(projects: Project[], orderedIds: string[]): Project[] {
+  if (projects.length === 0) return [];
+  if (orderedIds.length === 0) return projects;
+
+  const byId = new Map(projects.map((project) => [project.id, project]));
+  const sorted: Project[] = [];
+
+  for (const id of orderedIds) {
+    const project = byId.get(id);
+    if (!project) continue;
+    sorted.push(project);
+    byId.delete(id);
+  }
+  for (const project of byId.values()) {
+    sorted.push(project);
+  }
+  return sorted;
+}