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/components/ui/button.tsx

@@ -27,8 +27,12 @@ const variantMap: Record<string, "primary" | "secondary" | "danger" | "ghost"> =
   link: "ghost",
 };
 
-export function Button({ variant = "primary", size = "md", ...props }: ButtonProps) {
-  const mappedVariant = variantMap[variant] || variant;
+export function Button({
+  variant = "primary",
+  size = "md",
+  ...props
+}: ButtonProps): React.JSX.Element {
+  const mappedVariant = variantMap[variant] ?? variant;
   const mappedSize = size === "icon" ? "sm" : size;
 
   return (