build: migrate to ESLint 9 flat config

Replace legacy .eslintrc.json files with a flat eslint.config.js that
preserves the existing ruleset (eslint:recommended, typescript-eslint
recommended, angular-eslint recommended + inline templates, template
recommended + accessibility, and the lib selector rules).

- eslint ^8.57.0 -> ^9.39.4
- @typescript-eslint/* v7 packages -> typescript-eslint ^8.61.0
- individual @angular-eslint/* packages -> angular-eslint 21.4.0 meta
  package (resolves the previously unsupported combination of
  @angular-eslint 21 with ESLint 8)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: L-X-T

.eslintrc.json

@@ -0,0 +1,42 @@
+// @ts-check
+const eslint = require('@eslint/js');
+const tseslint = require('typescript-eslint');
+const angular = require('angular-eslint');
+
+module.exports = tseslint.config(
+  {
+    files: ['**/*.ts'],
+    extends: [
+      eslint.configs.recommended,
+      ...tseslint.configs.recommended,
+      ...angular.configs.tsRecommended,
+    ],
+    processor: angular.processInlineTemplates,
+    rules: {
+      '@angular-eslint/directive-selector': [
+        'error',
+        {
+          type: 'attribute',
+          prefix: 'lib',
+          style: 'camelCase',
+        },
+      ],
+      '@angular-eslint/component-selector': [
+        'error',
+        {
+          type: 'element',
+          prefix: 'lib',
+          style: 'kebab-case',
+        },
+      ],
+    },
+  },
+  {
+    files: ['**/*.html'],
+    extends: [
+      ...angular.configs.templateRecommended,
+      ...angular.configs.templateAccessibility,
+    ],
+    rules: {},
+  }
+);