feat: add sidebarMode and activeDmUserId to UI store

Extends UiState with sidebarMode ("channels" | "dms") and
activeDmUserId (number | null). Adds setSidebarMode (which auto-clears
activeDmUserId when switching back to channels) and setActiveDmUser
actions. All three new tests pass (TDD: RED → GREEN).

Client/tauri-client/src/stores/ui.store.ts

@@ -15,6 +15,8 @@ export interface UiState {
   readonly transientError: string | null;
   readonly persistentError: string | null;
   readonly collapsedCategories: ReadonlySet<string>;
+  readonly sidebarMode: "channels" | "dms";
+  readonly activeDmUserId: number | null;
 }
 
 const INITIAL_STATE: UiState = {
@@ -27,6 +29,8 @@ const INITIAL_STATE: UiState = {
   transientError: null,
   persistentError: null,
   collapsedCategories: new Set(),
+  sidebarMode: "channels",
+  activeDmUserId: null,
 };
 
 export const uiStore = createStore<UiState>(INITIAL_STATE);
@@ -130,3 +134,21 @@ export function toggleCategory(category: string): void {
 export function isCategoryCollapsed(category: string): boolean {
   return uiStore.select((s) => s.collapsedCategories.has(category));
 }
+
+/** Switch the sidebar between channel mode and DM mode.
+ *  Switching back to "channels" clears the active DM user. */
+export function setSidebarMode(mode: "channels" | "dms"): void {
+  uiStore.setState((prev) => ({
+    ...prev,
+    sidebarMode: mode,
+    activeDmUserId: mode === "channels" ? null : prev.activeDmUserId,
+  }));
+}
+
+/** Set the currently active DM conversation user ID. */
+export function setActiveDmUser(userId: number | null): void {
+  uiStore.setState((prev) => ({
+    ...prev,
+    activeDmUserId: userId,
+  }));
+}

Client/tauri-client/tests/unit/ui.store.test.ts

@@ -10,6 +10,8 @@ import {
   setTheme,
   toggleCategory,
   isCategoryCollapsed,
+  setSidebarMode,
+  setActiveDmUser,
 } from "../../src/stores/ui.store";
 
 function resetStore(): void {
@@ -23,6 +25,8 @@ function resetStore(): void {
     transientError: null,
     persistentError: null,
     collapsedCategories: new Set<string>(),
+    sidebarMode: "channels" as const,
+    activeDmUserId: null,
   }));
 }
 
@@ -190,4 +194,32 @@ describe("ui store", () => {
       expect(listener).not.toHaveBeenCalled();
     });
   });
+
+  describe("sidebar mode", () => {
+    beforeEach(() => {
+      uiStore.setState((prev) => ({
+        ...prev,
+        sidebarMode: "channels",
+        activeDmUserId: null,
+      }));
+    });
+
+    it("defaults to channels mode", () => {
+      expect(uiStore.getState().sidebarMode).toBe("channels");
+    });
+
+    it("switches to DM mode with user ID", () => {
+      setSidebarMode("dms");
+      setActiveDmUser(42);
+      const state = uiStore.getState();
+      expect(state.sidebarMode).toBe("dms");
+      expect(state.activeDmUserId).toBe(42);
+    });
+
+    it("clears DM user when switching back to channels", () => {
+      setActiveDmUser(42);
+      setSidebarMode("channels");
+      expect(uiStore.getState().activeDmUserId).toBeNull();
+    });
+  });
 });