fix: Resolve all ESLint errors and warnings in web package

Fixes all 542 ESLint problems in the web package to achieve 0 errors and 0 warnings.

Changes:
- Fixed 144 issues: nullish coalescing, return types, unused variables
- Fixed 118 issues: unnecessary conditions, type safety, template literals
- Fixed 79 issues: non-null assertions, unsafe assignments, empty functions
- Fixed 67 issues: explicit return types, promise handling, enum comparisons
- Fixed 45 final warnings: missing return types, optional chains
- Fixed 25 typecheck-related issues: async/await, type assertions, formatting
- Fixed JSX.Element namespace errors across 90+ files

All Quality Rails violations resolved. Lint and typecheck both pass with 0 problems.

Files modified: 118 components, tests, hooks, and utilities

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/src/lib/hooks/useLayout.ts

@@ -13,10 +13,26 @@ const DEFAULT_LAYOUT_NAME = "default";
  */
 const WORKSPACE_KEY = "mosaic-workspace-id";
 
+interface UseLayoutReturn {
+  layouts: Record<string, LayoutConfig>;
+  currentLayout: LayoutConfig | undefined;
+  currentLayoutId: string;
+  isLoading: boolean;
+  updateLayout: (layoutItems: WidgetPlacement[]) => void;
+  addWidget: (widget: WidgetPlacement) => void;
+  removeWidget: (widgetId: string) => void;
+  updateWidget: (widgetId: string, updates: Partial<WidgetPlacement>) => void;
+  createLayout: (name: string) => string;
+  deleteLayout: (layoutId: string) => void;
+  renameLayout: (layoutId: string, name: string) => void;
+  switchLayout: (layoutId: string) => void;
+  resetLayout: () => void;
+}
+
 /**
  * Hook to manage widget layout state
  */
-export function useLayout() {
+export function useLayout(): UseLayoutReturn {
   const [layouts, setLayouts] = useState<Record<string, LayoutConfig>>({});
   const [currentLayoutId, setCurrentLayoutId] = useState<string>(DEFAULT_LAYOUT_NAME);
   const [isLoading, setIsLoading] = useState(true);
@@ -26,7 +42,7 @@ export function useLayout() {
     try {
       const stored = localStorage.getItem(STORAGE_KEY);
       if (stored) {
-        const parsed = JSON.parse(stored);
+        const parsed = JSON.parse(stored) as Record<string, LayoutConfig>;
         setLayouts(parsed);
       }
 
@@ -62,7 +78,7 @@ export function useLayout() {
         ...prev,
         [currentLayoutId]: {
           id: currentLayoutId,
-          name: prev[currentLayoutId]?.name || "My Layout",
+          name: prev[currentLayoutId]?.name ?? "My Layout",
           layout: layoutItems,
         },
       }));
@@ -133,7 +149,7 @@ export function useLayout() {
   );
 
   const createLayout = useCallback((name: string) => {
-    const id = `layout-${Date.now()}`;
+    const id = `layout-${String(Date.now())}`;
     setLayouts((prev) => ({
       ...prev,
       [id]: {
@@ -149,11 +165,11 @@ export function useLayout() {
   const deleteLayout = useCallback(
     (layoutId: string) => {
       setLayouts((prev) => {
-        const { [layoutId]: deleted, ...rest } = prev;
+        const { [layoutId]: _deleted, ...rest } = prev;
         // If we deleted the current layout, switch to default
         if (layoutId === currentLayoutId) {
           const remainingIds = Object.keys(rest);
-          setCurrentLayoutId(remainingIds[0] || DEFAULT_LAYOUT_NAME);
+          setCurrentLayoutId(remainingIds[0] ?? DEFAULT_LAYOUT_NAME);
         }
         return rest;
       });