feat: integrate Matrix chat (Option 2 - credentials stored in Supabase)

- Add matrix-js-sdk, marked, highlight.js, twemoji, @tanstack/svelte-virtual deps
- Copy Matrix core layer: /matrix/, /stores/matrix.ts, /cache/, /services/
- Copy Matrix components: matrix/, message/, chat-layout/, chat-settings/
- Copy UI components: EmojiPicker, Twemoji, ImagePreviewModal, VirtualList
- Copy utils: emojiData, twemoji, twemojiGlobal
- Replace lucide-svelte with Material Symbols in SyncRecoveryBanner
- Extend Avatar with xs size and status indicator prop
- Fix ui.ts store conflict: re-export toasts from toast.svelte.ts
- Add migration 020_matrix_credentials for storing Matrix tokens per user/org
- Add /api/matrix-credentials endpoint (GET/POST/DELETE)
- Create [orgSlug]/chat page with Matrix login form + full chat UI
- Add Chat to sidebar navigation

src/lib/stores/ui.ts

@@ -0,0 +1,55 @@
+/**
+ * UI Stores
+ * 
+ * Re-exports toasts from the main toast store for backward compatibility
+ * with Matrix components, plus Matrix-specific UI state.
+ */
+
+import { writable } from 'svelte/store';
+
+// Re-export toasts so Matrix components can import from '$lib/stores/ui'
+export { toasts } from '$lib/stores/toast.svelte';
+
+// ============================================================================
+// Chat Layout State
+// ============================================================================
+
+export const sidebarOpen = writable(true);
+export const membersPanelOpen = writable(false);
+
+// ============================================================================
+// Chat Modals
+// ============================================================================
+
+export type ModalType =
+  | 'none'
+  | 'createRoom'
+  | 'roomSettings'
+  | 'roomMembers'
+  | 'userProfile'
+  | 'settings';
+
+export const activeModal = writable<ModalType>('none');
+export const modalData = writable<any>(null);
+
+export function openModal(type: ModalType, data?: any): void {
+  activeModal.set(type);
+  modalData.set(data ?? null);
+}
+
+export function closeModal(): void {
+  activeModal.set('none');
+  modalData.set(null);
+}
+
+// ============================================================================
+// Loading States
+// ============================================================================
+
+export const isLoading = writable(false);
+export const loadingMessage = writable<string | null>(null);
+
+export function setLoading(loading: boolean, message?: string): void {
+  isLoading.set(loading);
+  loadingMessage.set(message ?? null);
+}