feat(assistant): implement spec 021 chat assistant flow with semantic contracts

frontend/src/lib/stores/__tests__/assistantChat.test.js

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+// [DEF:frontend.src.lib.stores.__tests__.assistantChat:Module]
+// @TIER: STANDARD
+// @SEMANTICS: test, store, assistant, toggle, conversation
+// @PURPOSE: Validate assistant chat store visibility and conversation binding transitions.
+// @LAYER: UI Tests
+// @RELATION: DEPENDS_ON -> assistantChatStore
+// @INVARIANT: Each test starts from default closed state.
+
+import { describe, it, expect, beforeEach } from 'vitest';
+import { get } from 'svelte/store';
+import {
+  assistantChatStore,
+  toggleAssistantChat,
+  openAssistantChat,
+  closeAssistantChat,
+  setAssistantConversationId,
+} from '../assistantChat.js';
+
+// [DEF:assistantChatStore_tests:Function]
+// @TIER: STANDARD
+// @PURPOSE: Group store unit scenarios for assistant panel behavior.
+// @PRE: Store can be reset to baseline state in beforeEach hook.
+// @POST: Open/close/toggle/conversation transitions are validated.
+describe('assistantChatStore', () => {
+  beforeEach(() => {
+    assistantChatStore.set({
+      isOpen: false,
+      conversationId: null,
+    });
+  });
+
+  it('should open assistant panel', () => {
+    openAssistantChat();
+    const state = get(assistantChatStore);
+    expect(state.isOpen).toBe(true);
+  });
+
+  it('should close assistant panel', () => {
+    openAssistantChat();
+    closeAssistantChat();
+    const state = get(assistantChatStore);
+    expect(state.isOpen).toBe(false);
+  });
+
+  it('should toggle assistant panel state', () => {
+    toggleAssistantChat();
+    expect(get(assistantChatStore).isOpen).toBe(true);
+    toggleAssistantChat();
+    expect(get(assistantChatStore).isOpen).toBe(false);
+  });
+
+  it('should set conversation id', () => {
+    setAssistantConversationId('conv-123');
+    const state = get(assistantChatStore);
+    expect(state.conversationId).toBe('conv-123');
+  });
+});
+// [/DEF:assistantChatStore_tests:Function]
+// [/DEF:frontend.src.lib.stores.__tests__.assistantChat:Module]

frontend/src/lib/stores/assistantChat.js

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+// [DEF:assistantChat:Store]
+// @TIER: STANDARD
+// @SEMANTICS: assistant, store, ui-state, conversation
+// @PURPOSE: Control assistant chat panel visibility and active conversation binding.
+// @LAYER: UI
+// @RELATION: BINDS_TO -> AssistantChatPanel
+// @INVARIANT: conversationId persists while panel toggles unless explicitly reset.
+//
+// @UX_STATE: Closed -> Panel hidden.
+// @UX_STATE: Open -> Panel visible and interactive.
+
+import { writable } from 'svelte/store';
+
+const initialState = {
+  isOpen: false,
+  conversationId: null,
+};
+
+export const assistantChatStore = writable(initialState);
+
+// [DEF:toggleAssistantChat:Function]
+// @PURPOSE: Toggle assistant panel visibility.
+// @PRE: Store is initialized.
+// @POST: isOpen value inverted.
+export function toggleAssistantChat() {
+  assistantChatStore.update((state) => {
+    const next = { ...state, isOpen: !state.isOpen };
+    console.log(`[assistantChat][${next.isOpen ? 'Open' : 'Closed'}] toggleAssistantChat`);
+    return next;
+  });
+}
+// [/DEF:toggleAssistantChat:Function]
+
+// [DEF:openAssistantChat:Function]
+// @PURPOSE: Open assistant panel.
+// @PRE: Store is initialized.
+// @POST: isOpen = true.
+export function openAssistantChat() {
+  assistantChatStore.update((state) => {
+    const next = { ...state, isOpen: true };
+    console.log('[assistantChat][Open] openAssistantChat');
+    return next;
+  });
+}
+// [/DEF:openAssistantChat:Function]
+
+// [DEF:closeAssistantChat:Function]
+// @PURPOSE: Close assistant panel.
+// @PRE: Store is initialized.
+// @POST: isOpen = false.
+export function closeAssistantChat() {
+  assistantChatStore.update((state) => {
+    const next = { ...state, isOpen: false };
+    console.log('[assistantChat][Closed] closeAssistantChat');
+    return next;
+  });
+}
+// [/DEF:closeAssistantChat:Function]
+
+// [DEF:setAssistantConversationId:Function]
+// @PURPOSE: Bind current conversation id in UI state.
+// @PRE: conversationId is string-like identifier.
+// @POST: store.conversationId updated.
+export function setAssistantConversationId(conversationId) {
+  assistantChatStore.update((state) => {
+    console.log('[assistantChat][ConversationBound] setAssistantConversationId');
+    return { ...state, conversationId };
+  });
+}
+// [/DEF:setAssistantConversationId:Function]
+// [/DEF:assistantChat:Store]