v0.5.2 — Add ESLint + typescript-eslint, fix lint errors

Add flat-config ESLint with typescript-eslint recommended rules,
fix all lint errors (unused imports/vars, empty catches, Function types,
prefer-const, useless assignments), and rename stale kpc- CSS class to kcc-.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/preload/translator.ts

@@ -138,6 +138,7 @@ const SYSTEM_PATTERNS = [
 // ── Pre-translation filtering ──
 
 function isLatinOnly(text: string): boolean {
+  // eslint-disable-next-line no-control-regex
   return /^[\x00-\x7F\u00C0-\u024F\u1E00-\u1EFF\s\d.,!?;:'"()\-/@#$%^&*+=~`[\]{}|\\<>]+$/u.test(text);
 }