refactor: state.js → mutable object to fix 'assignment to constant' errors

ES module imports are read-only live bindings — can't reassign
imported let/const variables. Replace individual 'export let' with
single 'export const state = {...}' mutable object.

All modules updated: import { state } from './state.js'
All state access changed to state.xxx pattern.

Fixes cascade of 'Assignment to constant variable' errors.

frontend/js/state.js

@@ -1,55 +1,59 @@
-/* ObsiGate — Shared application state */
-export const APP_VERSION = "1.5.0";
+/* ObsiGate — Shared mutable application state.
+   Use state.xxx to read/write any value. ES module imports are read-only,
+   so we export a single mutable object instead of individual let bindings. */
+export const state = {
+  APP_VERSION: "1.5.0",
 
-// Core navigation state
-export let currentVault = null;
-export let currentPath = null;
-export let allVaults = [];
-export let selectedContextVault = "all";
+  // Core navigation
+  currentVault: null,
+  currentPath: null,
+  allVaults: [],
+  selectedContextVault: "all",
 
-// Search state
-export let searchTimeout = null;
-export let searchAbortController = null;
-export let advancedSearchOffset = 0;
-export let advancedSearchTotal = 0;
-export let advancedSearchSort = "relevance";
-export let advancedSearchLastQuery = "";
-export let suggestAbortController = null;
-export let dropdownActiveIndex = -1;
-export let dropdownItems = [];
-export let currentSearchId = 0;
-export let selectedTags = [];
-export let searchCaseSensitive = false;
-export let searchWholeWord = false;
-export let searchRegex = false;
-export let searchFilterVisible = false;
+  // Search
+  searchTimeout: null,
+  searchAbortController: null,
+  advancedSearchOffset: 0,
+  advancedSearchTotal: 0,
+  advancedSearchSort: "relevance",
+  advancedSearchLastQuery: "",
+  suggestAbortController: null,
+  dropdownActiveIndex: -1,
+  dropdownItems: [],
+  currentSearchId: 0,
+  selectedTags: [],
+  searchCaseSensitive: false,
+  searchWholeWord: false,
+  searchRegex: false,
+  searchFilterVisible: false,
 
-// Search constants
-export const SEARCH_HISTORY_KEY = "obsigate_search_history";
-export const MAX_HISTORY_ENTRIES = 50;
-export const SUGGEST_DEBOUNCE_MS = 150;
-export const ADVANCED_SEARCH_LIMIT = 50;
-export const MIN_SEARCH_LENGTH = 2;
-export const SEARCH_TIMEOUT_MS = 30000;
+  // Search constants
+  SEARCH_HISTORY_KEY: "obsigate_search_history",
+  MAX_HISTORY_ENTRIES: 50,
+  SUGGEST_DEBOUNCE_MS: 150,
+  ADVANCED_SEARCH_LIMIT: 50,
+  MIN_SEARCH_LENGTH: 2,
+  SEARCH_TIMEOUT_MS: 30000,
 
-// Viewer state
-export let showingSource = false;
-export let cachedRawSource = null;
-export let editorView = null;
-export let editorVault = null;
-export let editorPath = null;
-export let fallbackEditorEl = null;
-export let _iconDebounceTimer = null;
+  // Viewer state
+  showingSource: false,
+  cachedRawSource: null,
+  editorView: null,
+  editorVault: null,
+  editorPath: null,
+  fallbackEditorEl: null,
+  _iconDebounceTimer: null,
 
-// Outline/TOC state
-export let outlineObserver = null;
-export let activeHeadingId = null;
-export let headingsCache = [];
-export let rightSidebarVisible = true;
-export let rightSidebarWidth = 280;
+  // Outline/TOC
+  outlineObserver: null,
+  activeHeadingId: null,
+  headingsCache: [],
+  rightSidebarVisible: true,
+  rightSidebarWidth: 280,
 
-// Sidebar state
-export let sidebarFilterCaseSensitive = false;
-export let activeSidebarTab = "vaults";
-export let filterDebounce = null;
-export let vaultSettings = {};
+  // Sidebar
+  sidebarFilterCaseSensitive: false,
+  activeSidebarTab: "vaults",
+  filterDebounce: null,
+  vaultSettings: {},
+};