Update app icon and Discord Rich Presence

Replace old KPC placeholder icon with new crosshair design. Generate
multi-size .ico (16-256px) and .png from 1024x1024 source. Update
Discord RPC to new application ID (1477679025248800982).

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

src/main/discord-rpc.ts

@@ -1,7 +1,7 @@
 import { Socket } from 'net';
 import { electronLog } from './logger';
 
-const DISCORD_CLIENT_ID = '1474451871694323975';
+const DISCORD_CLIENT_ID = '1477679025248800982';
 
 // Discord IPC opcodes
 const OP_HANDSHAKE = 0;