fix: Improve index.html missing error handling and fix ESLint config

When index.html is not found during SPA fallback, the code now returns a
proper 500 server error with a clear message instead of a misleading 404.
This correctly indicates a build/deployment error rather than a user request
error.

Also fixes pre-existing ESLint configuration issues:
- Configured separate TypeScript project configs for backend and frontend
  code to resolve parsing errors in src/webui/static files
- Added test files to tsconfig.json include array
- Replaced conditional require() with proper ES6 top-level import
  for Windows readline handling

Changes:
- WebUIManager: Return AppError with ErrorCode.CONFIG_INVALID when
  index.html is missing, including contextual debugging information
- WebUIManager: Added defensive documentation explaining why
  path.extname() check is safe (app doesn't use client-side routing)
- eslint.config.mjs: Split configuration for backend (CommonJS) and
  frontend (ES modules) with appropriate tsconfig references
- tsconfig.json: Added test files to include array
- index.ts: Replaced conditional require('readline') with top-level import

Fixes #10 comment #3 (Gemini Code Assist review)

Author: GhostTypes

eslint.config.mjs

@@ -6,11 +6,19 @@ import globals from 'globals';
 export default tseslint.config(
   eslint.configs.recommended,
   ...tseslint.configs.recommended,
+
+  // Backend configuration - uses main tsconfig.json
   {
-    files: ['**/*.ts', '**/*.tsx'],
+    files: ['src/**/*.ts', '**/*.ts'],
+    ignores: [
+      'src/webui/static/**/*.ts',
+      'dist/**',
+      'node_modules/**',
+      '.dependencies/**'
+    ],
     languageOptions: {
       ecmaVersion: 2022,
-      sourceType: 'module',
+      sourceType: 'commonjs',
       globals: {
         ...globals.node,
       },
@@ -29,7 +37,29 @@ export default tseslint.config(
       ],
     },
   },
+
+  // Frontend configuration - uses webui static tsconfig
   {
-    ignores: ['dist/**', 'node_modules/**', '.dependencies/**'],
+    files: ['src/webui/static/**/*.ts'],
+    languageOptions: {
+      ecmaVersion: 2020,
+      sourceType: 'module',
+      globals: {
+        ...globals.browser,
+      },
+      parserOptions: {
+        project: './src/webui/static/tsconfig.json',
+      },
+    },
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'warn',
+      '@typescript-eslint/no-unused-vars': [
+        'error',
+        {
+          argsIgnorePattern: '^_',
+          varsIgnorePattern: '^_',
+        },
+      ],
+    },
   }
 );

src/index.ts

@@ -28,6 +28,7 @@ import { getRtspStreamService } from './services/RtspStreamService';
 import { initializeSpoolmanIntegrationService } from './services/SpoolmanIntegrationService';
 import { getSavedPrinterService } from './services/SavedPrinterService';
 import { parseHeadlessArguments, validateHeadlessConfig } from './utils/HeadlessArguments';
+import * as readline from 'readline';
 import { withTimeout, createHardDeadline } from './utils/ShutdownTimeout';
 import type { HeadlessConfig, PrinterSpec } from './utils/HeadlessArguments';
 import type { PrinterDetails, PrinterClientType } from './types/printer';
@@ -294,7 +295,6 @@ function setupSignalHandlers(): void {
 
   // Windows-specific: Handle process termination
   if (process.platform === 'win32') {
-    const readline = require('readline');
     const rl = readline.createInterface({
       input: process.stdin,
       output: process.stdout

src/webui/server/WebUIManager.ts

@@ -232,6 +232,10 @@ export class WebUIManager extends EventEmitter {
 
     // SPA fallback - serve index.html for non-API routes that don't match static files
     // This enables client-side routing in the WebUI
+
+    // NOTE: Using path.extname() is safe here because this app does NOT use client-side routing.
+    // All UI state is managed via DOM manipulation, not URL routes. If client-side routing
+    // is added in the future, this should be changed to use Accept header detection instead.
     this.expressApp.get('/*splat', (req, res, next) => {
       // Skip if this looks like a file request with extension (handled by static middleware)
       if (path.extname(req.path) && req.path !== '/') {
@@ -249,7 +253,12 @@ export class WebUIManager extends EventEmitter {
       if (fs.existsSync(indexPath)) {
         res.sendFile(indexPath);
       } else {
-        next();
+        // index.html is missing - this indicates a build/deployment error
+        next(new AppError(
+          'WebUI application files not found. Please ensure the application has been built properly.',
+          ErrorCode.CONFIG_INVALID,
+          { indexPath, staticPath: this.webUIStaticPath }
+        ));
       }
     });
 

tsconfig.json

@@ -28,7 +28,9 @@
     "src/services/**/*.ts",
     "src/types/**/*.ts",
     "src/utils/**/*.ts",
-    "src/webui/**/*.ts"
+    "src/webui/**/*.ts",
+    "src/__tests__/**/*.ts",
+    "**/*.test.ts"
   ],
   "exclude": [
     "node_modules",