feat(FN-3398): update desktop README with multi-project setup and troublesh

The merge lands Step 7 of FN-3398, adding documentation and delivery artifacts across the mobile and desktop packages with updated READMEs, mobile-specific docs, and architecture references.

Fusion-Task-Id: FN-3398

packages/desktop/src/__tests__/shell-settings.test.ts

@@ -0,0 +1,72 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const mockState = vi.hoisted(() => ({
+  content: new Map<string, string>(),
+}));
+
+vi.mock("electron", () => ({
+  app: {
+    getPath: vi.fn(() => "/tmp/fusion"),
+  },
+}));
+
+vi.mock("node:fs/promises", () => ({
+  readFile: vi.fn(async (path: string) => {
+    const value = mockState.content.get(path);
+    if (!value) {
+      const err = new Error("ENOENT") as NodeJS.ErrnoException;
+      err.code = "ENOENT";
+      throw err;
+    }
+    return value;
+  }),
+  writeFile: vi.fn(async (path: string, value: string) => {
+    mockState.content.set(path, value);
+  }),
+  rename: vi.fn(async (from: string, to: string) => {
+    const value = mockState.content.get(from) ?? "";
+    mockState.content.set(to, value);
+  }),
+}));
+
+describe("shell-settings", () => {
+  beforeEach(() => {
+    mockState.content.clear();
+    vi.resetModules();
+  });
+
+  it("returns defaults when file missing", async () => {
+    const { readShellSettings } = await import("../shell-settings.ts");
+    await expect(readShellSettings()).resolves.toEqual({
+      desktopMode: "remote",
+      activeProfileId: null,
+      profiles: [],
+    });
+  });
+
+  it("writes and reads persisted settings", async () => {
+    const { writeShellSettings, readShellSettings } = await import("../shell-settings.ts");
+
+    await writeShellSettings({
+      desktopMode: "local",
+      activeProfileId: "p1",
+      profiles: [
+        {
+          id: "p1",
+          name: "Local",
+          serverUrl: "http://127.0.0.1",
+          authToken: null,
+          createdAt: "2026-01-01T00:00:00.000Z",
+          updatedAt: "2026-01-01T00:00:00.000Z",
+          lastUsedAt: null,
+        },
+      ],
+    });
+
+    await expect(readShellSettings()).resolves.toMatchObject({
+      desktopMode: "local",
+      activeProfileId: "p1",
+      profiles: [{ id: "p1" }],
+    });
+  });
+});