feat(web): add shift-click multi-select for sidebar threads

apps/web/src/components/Sidebar.logic.test.ts

@@ -1,6 +1,10 @@
 import { describe, expect, it } from "vitest";
 
-import { hasUnseenCompletion, resolveThreadStatusPill } from "./Sidebar.logic";
+import {
+  hasUnseenCompletion,
+  resolveThreadStatusPill,
+  shouldClearThreadSelectionOnMouseDown,
+} from "./Sidebar.logic";
 
 function makeLatestTurn(overrides?: {
   completedAt?: string | null;
@@ -30,6 +34,34 @@ describe("hasUnseenCompletion", () => {
   });
 });
 
+describe("shouldClearThreadSelectionOnMouseDown", () => {
+  it("preserves selection for thread items", () => {
+    const child = {
+      closest: (selector: string) =>
+        selector.includes("[data-thread-item]") ? ({} as Element) : null,
+    } as unknown as HTMLElement;
+
+    expect(shouldClearThreadSelectionOnMouseDown(child)).toBe(false);
+  });
+
+  it("preserves selection for thread list toggle controls", () => {
+    const selectionSafe = {
+      closest: (selector: string) =>
+        selector.includes("[data-thread-selection-safe]") ? ({} as Element) : null,
+    } as unknown as HTMLElement;
+
+    expect(shouldClearThreadSelectionOnMouseDown(selectionSafe)).toBe(false);
+  });
+
+  it("clears selection for unrelated sidebar clicks", () => {
+    const unrelated = {
+      closest: () => null,
+    } as unknown as HTMLElement;
+
+    expect(shouldClearThreadSelectionOnMouseDown(unrelated)).toBe(true);
+  });
+});
+
 describe("resolveThreadStatusPill", () => {
   const baseThread = {
     interactionMode: "plan" as const,

apps/web/src/components/Sidebar.logic.ts

@@ -1,6 +1,9 @@
 import type { Thread } from "../types";
 import { findLatestProposedPlan, isLatestTurnSettled } from "../session-logic";
 
+export const THREAD_SELECTION_SAFE_SELECTOR =
+  "[data-thread-item], [data-thread-selection-safe]";
+
 export interface ThreadStatusPill {
   label:
     | "Working"
@@ -30,6 +33,11 @@ export function hasUnseenCompletion(thread: ThreadStatusInput): boolean {
   return completedAt > lastVisitedAt;
 }
 
+export function shouldClearThreadSelectionOnMouseDown(target: HTMLElement | null): boolean {
+  if (target === null) return true;
+  return !target.closest(THREAD_SELECTION_SAFE_SELECTOR);
+}
+
 export function resolveThreadStatusPill(input: {
   thread: ThreadStatusInput;
   hasPendingApprovals: boolean;