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/main/client-ui.ts

@@ -518,7 +518,7 @@ export const MATCHMAKER_SETTINGS_CSS = `
 `;
 
 export const TRANSLATOR_CSS = `
-.kpc-translation {
+.kcc-translation {
   color: #88ff88;
   font-style: italic;
   margin-left: 8px;